Read The Docs integration: fixing class reference in Project

The case of the class name was wrong.

This was found by the (not present yet in GitLab 14.2.6) Integrations
API RSpec tests on GitLab 14.4, affecting apparently only the DELETE
call. Impact in Heptapod 0.25 is not clear, but the fact remains that
the case was wrong.

branch : heptapod-oldstable
4 jobs for !307 with topic/heptapod-oldstable/readthedocs in 10 minutes and 37 seconds (queued for 3 seconds)
latest merge request