[Docs][Github] Say '--delete-branch' instead of '--delete' and '--delete branch'

The documentation for how to use "gh pr merge" was broken.
In one place it said '--delete-branch' (which seem to be correct for
that option). But in second place it said '--delete branch' and in a
third place it just said '--delete'.
Make sure '--delete-branch' is used in all three places.

Author: bjope

llvm/docs/GitHub.rst

@@ -156,7 +156,7 @@ merge:
 ::
 
   git push -f
-  gh pr merge --squash --delete branch
+  gh pr merge --squash --delete-branch
 
 This force push may ask if you intend to push hundreds, or potentially
 thousands of patches (depending on how long it's been since your pull request
@@ -248,7 +248,7 @@ checks:
   git push origin my_change -f
 
   # Now merge it
-  gh pr merge --squash --delete
+  gh pr merge --squash --delete-branch
 
 
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