Documentation/git-am.txt: Pass -r in the example invocation of rm -f …

….dotest

Signed-off-by: Bryan Donlan <bdonlan@fushizen.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -138,7 +138,7 @@ aborts in the middle,.  You can recover from this in one of two ways:
 
 The command refuses to process new mailboxes while `.dotest`
 directory exists, so if you decide to start over from scratch,
-run `rm -f .dotest` before running the command with mailbox
+run `rm -f -r .dotest` before running the command with mailbox
 names.