cmd/nm: put the -S flag in the usage message

The -S flag reports symbol size, but is missing from the usage message.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/13660046

src/cmd/nm/doc.go

@@ -17,7 +17,7 @@ This implementation adds the flag -S, which prints each symbol's size
 in decimal after its address.
 
 Usage:
-	go tool nm [-aghnsTu] file
+	go tool nm [-aghnsSTu] file
 
 */
 package main

src/cmd/nm/nm.c

@@ -72,7 +72,7 @@ void	zenter(Sym*);
 void
 usage(void)
 {
-	fprint(2, "usage: nm [-aghnsTu] file ...\n");
+	fprint(2, "usage: nm [-aghnsSTu] file ...\n");
 	exits("usage");
 }