Documentation Fix: Fixed value of `CI_PIPELINE_SOURCE` for multi-project...

Documentation Fix: Fixed value of `CI_PIPELINE_SOURCE` for multi-project pipelines created with `CI_JOB_TOKEN`.

It should be `pipeline` and not `pipelines`. The correct value is already mentioned in https://docs.gitlab.com/ee/ci/variables/predefined_variables.html

doc/ci/yaml/README.md

@@ -1275,7 +1275,7 @@ check the value of the `$CI_PIPELINE_SOURCE` variable.
 | `schedule`                    | For [scheduled pipelines](../pipelines/schedules.md).                                                                                                                                                                                                                                                                                                 |
 | `api`                         | For pipelines triggered by the [pipelines API](../../api/pipelines.md#create-a-new-pipeline).                                                                                                                                                                                                                                                         |
 | `external`                    | When using CI services other than GitLab.                                                                                                                                                                                                                                                                                                             |
-| `pipelines`                   | For multi-project pipelines created by [using the API with `CI_JOB_TOKEN`](../triggers/README.md#when-used-with-multi-project-pipelines).                                                                                                                                                                                                             |
+| `pipeline`                   | For multi-project pipelines created by [using the API with `CI_JOB_TOKEN`](../triggers/README.md#when-used-with-multi-project-pipelines).                                                                                                                                                                                                             |
 | `chat`                        | For pipelines created by using a [GitLab ChatOps](../chatops/README.md) command.                                                                                                                                                                                                                                                                      |
 | `webide`                      | For pipelines created by using the [WebIDE](../../user/project/web_ide/index.md).                                                                                                                                                                                                                                                                     |
 | `merge_request_event`         | For pipelines created when a merge request is created or updated. Required to enable [merge request pipelines](../merge_request_pipelines/index.md), [merged results pipelines](../merge_request_pipelines/pipelines_for_merged_results/index.md), and [merge trains](../merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md). |