Fix usage string.

Change-Id: Iaa99a1bb8bb6998082f3553689b5b812a6471514
diff --git a/mkbootimg/mkbootimg.c b/mkbootimg/mkbootimg.c
index bdc572e..47ab5f1 100644
--- a/mkbootimg/mkbootimg.c
+++ b/mkbootimg/mkbootimg.c
@@ -64,6 +64,7 @@
             "       [ --cmdline <kernel-commandline> ]\n"
             "       [ --board <boardname> ]\n"
             "       [ --base <address> ]\n"
+            "       [ --pagesize <pagesize> ]\n"
             "       -o|--output <filename>\n"
             );
     return 1;