Fix file extension for `serverless.yaml` in error dialog

What does this MR do?

Fix file extension for serverless.yaml in error dialog The error message incorrectly refers to serverless.yml, this Merge Request corrects it to serverless.yaml

See note after point 2 on https://docs.gitlab.com/ee/user/project/clusters/serverless/#deploying-functions

What are the relevant issue numbers?

None that I could find.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer