arc docs: correct the instructions on landing a revision

Summary:
I incorrectly assumed that `arc land --revision D123` would pull D123
from Phabricator and land that. This is incorrect - arc land lands the
locally checked out work (or another local branch if a ref is given) in git.
`arc patch D123` then `arc land` is the correct process.

Test Plan: Confirm the docs are correct and check grammer.

Reviewers: mcote

Reviewed By: mcote

Differential Revision: https://phabricator.services.mozilla.com/D190

phabricator-user.rst

@@ -386,7 +386,8 @@ and to communicate what they should do next.  These actions include:
 Landing Patches
 ***************
 
-Landing to git repositories is simple: ``arc land --revision D123``. arc
+Landing to git repositories is simple: ``arc patch D123`` (assuming that the
+most recent version is not already checked out locally) then ``arc land``. arc
 will apply the revision on top of the master branch and push to the remote
 origin, assuming you have permission. You can override the "master" branch with
 the ``--onto`` flag and you can override the remote origin with the ``--remote``