Fix typo in `RENOVATE_X_GITLAB_AUTO_MERGEABLE_CHECK_ATTEMPS`

https://docs.renovatebot.com/self-hosted-experimental/#renovate_x_gitlab_auto_mergeable_check_attemps should be:

-RENOVATE_X_GITLAB_AUTO_MERGEABLE_CHECK_ATTEMPS
+RENOVATE_X_GITLAB_AUTO_MERGEABLE_CHECK_ATTEMPTS