Add missing URL to snippet feature flag yml

What does this MR do?

In a recent MR I introduced a new feature flag, but forgot to fill in the introduced_by value for the new feature flag config YML, as described by https://docs.gitlab.com/ee/development/feature_flags/development.html#feature-flag-definition-and-validation

Although not required by our process, I think it's useful information to have, especially because the next steps are not scheduled during the next milestone or so.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content