[llvm] [llvm][docs] 2 small fixes to GitHub guide (PR #134620)

[David Spickett via llvm-commits]

================
@@ -219,7 +218,7 @@ commonly used first:
 Branches
 ========
 
-It is possible to create branches that start with `users/<username>/`, however this is
+It is possible to create branches in `llvm/llvm-project/` that start with `users/<username>/`, however this is
----------------
DavidSpickett wrote:

I did at first but then I thought well, that's extra diff, but I'll put it back.

https://github.com/llvm/llvm-project/pull/134620