fixed broken link to git cheat sheet

What does this MR do?

There is a broken link here: doc/topics/git/index.md I could not find the missing .pdf file in the repo, so I linked it to the online version for now: https://about.gitlab.com/images/press/git-cheat-sheet.pdf. Is this okay? Or should we add the .pdf to the repo somewhere and link to it internally?

Why was this MR needed?

Broken links make a bad impression.

Suggested reviewers/approvers/mergers

@leipert @iamphill @timzallmann @rymai @leipert @timzallmann @axil

Thanks!