update usage

src/cmd/diff/main.c

diff --git a/src/cmd/diff/main.c b/src/cmd/diff/main.c
index d1ed124..a80f76e 100644
--- a/src/cmd/diff/main.c
+++ b/src/cmd/diff/main.c
@@ -11,7 +11,7 @@
 static char *tmp[] = {"/tmp/diff1XXXXXXXXXXX", "/tmp/diff2XXXXXXXXXXX"};
 static int whichtmp;
 static char *progname;
-static char usage[] = "diff [ -efmnbwr ] file1 ... file2\n";
+static char usage[] = "diff [ -acefmnbwr ] file1 ... file2\n";
 
 static void
 rmtmpfiles(void)