doc/ref: update git cheat sheet link
doc/ref: update git cheat sheet link

Commit 00288d76159a061724137d132ea681a9fc89c8f6 updated the
link in /doc/ but missed this matching one.

Mokolea/git-scm.com,jnavila/gitscm-next,git/git-scm.com,jnavila/gitscm-next,git/git-scm.com,git/git-scm.com,jasonlong/git-scm.com,Mokolea/git-scm.com,jnavila/gitscm-next,mosoft521/gitscm-next,mosoft521/gitscm-next,jasonlong/git-scm.com,mosoft521/gitscm-next,Mokolea/git-scm.com,jasonlong/git-scm.com,jnavila/gitscm-next,git/git-scm.com,Mokolea/git-scm.com,mosoft521/gitscm-next,jasonlong/git-scm.com