Fix usage.
 [rsc] --rw-rw-r-- M 121854 glenda sys 4340 Oct 19 07:53 sys/src/cmd/diff/main.c
	/n/sourcesdump/2005/1019/plan9/sys/src/cmd/diff/main.c:11,17 - 
	/n/sourcesdump/2005/1020/plan9/sys/src/cmd/diff/main.c:11,17
	  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)