rebase: mention -r argument in synopsis

I'm guessing that we simply forgot to mention it there. I was
wondering if maybe `-r` was experimental as long as evolution is, but
that doesn't seem to be the case (`-r` is not marked experimental).

Differential Revision: https://phab.mercurial-scm.org/D8295

hgext/rebase.py

@@ -871,7 +871,7 b' class rebaseruntime(object):'
     + cmdutil.dryrunopts
     + cmdutil.formatteropts
     + cmdutil.confirmopts,
-    _(b'[-s REV | -b REV] [-d REV] [OPTION]'),
+    _(b'[-s REV | -b REV | [-r REV]...] [-d REV] [OPTION]...'),
     helpcategory=command.CATEGORY_CHANGE_MANAGEMENT,
 )
 def rebase(ui, repo, **opts):