(docs) Updated fields information for push mirror from GitLab to GitHub

What does this MR do?

Fix instructions for Setting up a push mirror from GitLab to GitHub (documentation only).

I found this part to not work/unclear:

2. Fill in the Git repository URL field, with the personal access token instead of a password. For example: https://<GitHubUsername>:<GitHubPersonalAccessToken>@github.com/group/project.git.
3. Click the Mirror repository button.

Placing only the https://<GitHubUsername>:<GitHubPersonalAccessToken>@github.com/group/project.git in the Git repository URL and clicking the Mirror repository button (with empty password field) resulted in this error:

2:Gitlab::Git::CommandError: fatal: could not read Password for '[FILTERED]@github.com': No such device or address

However, splitting out the access token from the repository URL and placing in the password field resulted in a successful sync.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer