[docs] Improve HowTo commit changes from git

Summary: As a novice here I tried to `git push` my changes for a while before figuring out the correct workflow which is described on other pages. This small change doesn't reduce redundancy between those pages, but at least readers can follow the links now.

Reviewers: Kokan, Jim

Reviewed By: Kokan, Jim

Subscribers: riccibruno, kiszk, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72077