Fix source link in Usage Ping documentation

This line in the Usage Ping documentation was correct when written,
but the referenced file changed location. This updates the reference
to refer to the new subdirectory, to fix the link in a way that matches
the convention on this documentation page.

doc/development/product_analytics/usage_ping.md

@@ -786,7 +786,7 @@ appear to be associated to any of the services running, since they all appear to
 WARNING:
 This feature is intended solely for internal GitLab use.
 
-In order to add data for aggregated metrics into Usage Ping payload you should add corresponding definition into  [`aggregated_metrics.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/usage_data_counters/aggregated_metrics.yml) file. Each aggregate definition includes following parts:
+In order to add data for aggregated metrics into Usage Ping payload you should add corresponding definition in [`aggregated_metrics`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/usage_data_counters/aggregated_metrics/). Each aggregate definition includes following parts:
 
 - name: unique name under which aggregate metric is added to Usage Ping payload
 - operator: operator that defines how aggregated metric data is counted. Available operators are: