git-am: fix type in its usage string

Signed-off-by: Jörg Sommer <joerg@alea.gnuu.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-am.sh b/git-am.sh
index 5f0f241..2ecebc4 100755
--- a/git-am.sh
+++ b/git-am.sh
@@ -14,7 +14,7 @@
 3,3way          allow fall back on 3way merging if needed
 s,signoff       add a Signed-off-by line to the commit message
 u,utf8          recode into utf8 (default)
-k,keep          pass -k flagg to git-mailinfo
+k,keep          pass -k flag to git-mailinfo
 whitespace=     pass it through git-apply
 C=              pass it through git-apply
 p=              pass it through git-apply