git-am.txt: advertise 'git am --abort' instead of 'rm .git/rebase-apply'

'git am --abort' is around for quite a long time now, and users should
normally not poke around inside the .git directory, yet the
documentation of 'git am' still recommends the following:

  ... if you decide to start over from scratch,
  run `rm -f -r .git/rebase-apply` ...

Suggest 'git am --abort' instead.

It's not quite the same as the original, because 'git am --abort' will
restore the original branch, while simply removing '.git/rebase-apply'
won't, but that's rather a thinko in the original wording, because
that won't actually "start over _from scratch_".

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -173,9 +173,9 @@ aborts in the middle.  You can recover from this in one of two ways:
   the index file to bring it into a state that the patch should
   have produced.  Then run the command with the '--resolved' option.
 
-The command refuses to process new mailboxes while the `.git/rebase-apply`
-directory exists, so if you decide to start over from scratch,
-run `rm -f -r .git/rebase-apply` before running the command with mailbox
+The command refuses to process new mailboxes until the current
+operation is finished, so if you decide to start over from scratch,
+run `git am --abort` before running the command with mailbox
 names.
 
 Before any patches are applied, ORIG_HEAD is set to the tip of the