[Doc] Update branch name in Phabricator documentation

[bader]

Summary

master -> main

[mehdi_amini]

Thanks!

llvm/docs/Phabricator.rst

 ``Differential Revision`` line to your commit message then it is recommended
 that you close the review manually. In the web UI, under "Leap Into Action" put
 the git revision number in the Comment, set the Action to "Close Revision" and
 click Submit.  Note the review must have been Accepted first.
 
 Committing someone's change from Phabricator
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-On a clean Git repository on an up to date ``master`` branch run the
+On a clean Git repository on an up to date ``main`` branch run the
 following (where ``<Revision>`` is the Phabricator review number):
 
 ::
 
   arc patch D<Revision>
 
 
 This will create a new branch called ``arcpatch-D<Revision>`` based on the
-current ``master`` and will create a commit corresponding to ``D<Revision>`` with a
+current ``main`` and will create a commit corresponding to ``D<Revision>`` with a
 commit message derived from information in the Phabricator review.
 
 Check you are happy with the commit message and amend it if necessary.
 For example, ensure the 'Author' property of the commit is set to the original author.
 You can use a command to correct the author property if it is incorrect:
 
 ::
 
   git commit --amend --author="John Doe <[e-mail]>"
 
 Then, make sure the commit is up-to-date, and commit it. This can be done by running
 the following:
 
 ::
 
-  git pull --rebase https://github.com/llvm/llvm-project.git master
+  git pull --rebase https://github.com/llvm/llvm-project.git main
   git show # Ensure the patch looks correct.
   ninja check-$whatever # Rerun the appropriate tests if needed.
-  git push https://github.com/llvm/llvm-project.git HEAD:master
+  git push https://github.com/llvm/llvm-project.git HEAD:main
 
 
 Abandoning a change
 -------------------
 
 If you decide you should not commit the patch, you should explicitly abandon
 the review so that reviewers don't think it is still open. In the web UI,
 scroll to the bottom of the page where normally you would enter an overall