Fix docs for lightweight tag creation via API

changelogs/unreleased/docs-17499-documentation-errors-about-creating-a-new-tag.yml

+---
+title: Fix docs for lightweight tag creation via API
+merge_request:
+author:
+type: other

doc/api/tags.md

@@ -131,7 +131,7 @@ Parameters:
   "message": null
 }
 ```
-The message will be `nil` when creating a lightweight tag otherwise
+The message will be `null` when creating a lightweight tag otherwise
 it will contain the annotation.
 
 In case of an error,