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

[Andrzej Warzyński via llvm-commits]

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@@ -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
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banach-space wrote:

[nit] Wrap at 80 cols?

Not sure whether that's the official format for these files. Suggesting for consistency with the existing stye.

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