Fix incorrect URL in GitLab.com integration sample

What does this MR do?

Documentation for MR !78556 (merged) mistakenly copied the custom instance URL from the custom GitLab integration sample to the GitLab.com integration sample. This was really confusing, since there are two GitLab instances at play here, and samples for GitLab.com and a custom GitLab instance were now identical except for the display name. This MR removes the URL from the GitLab.com sample again.

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