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

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>