Fix docstring for completion events transformer

[Vishwajit-Herma]

Fix the naming error docstring. ('forum' to 'completion')

Description: Describe in a couple of sentences what this PR adds

JIRA: Link to JIRA ticket

Dependencies: dependencies on other outstanding PRs, issues, etc.

Merge deadline: List merge deadline (if any)

Installation instructions: List any non-trivial installation
instructions.

Testing instructions:

1. Open page A
2. Do thing B
3. Expect C to happen
4. If D happened instead - check failed.

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.

Author concerns: List any concerns about this PR - inelegant
solutions, hacks, quick-and-dirty implementations, concerns about
migrations, etc.

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