[DOC] Fix reference for closed form linear GW

[kachayev]

Types of changes

Fix docstring formatting for reference.

Motivation and context / Related issue

Otherwise it split the name of the paper treating the second line as a new paragraph.

How has this been tested (if it applies)

n/a

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.