godoc: change -x to -src, update doc.go (missed in previous CL)

R=rsc
CC=golang-dev
https://golang.org/cl/384044

src/cmd/godoc/doc.go

@@ -23,6 +23,8 @@ Usage:
 The flags are:
 	-v
 		verbose mode
+	-src
+		print exported source in command-line mode
 	-tabwidth=4
 		width of tabs in units of spaces
 	-path=""

src/cmd/godoc/main.go

@@ -48,7 +48,7 @@ var (
 
 	// layout control
 	html   = flag.Bool("html", false, "print HTML in command-line mode")
-	genAST = flag.Bool("x", false, "print exported source in command-line mode")
+	genAST = flag.Bool("src", false, "print exported source in command-line mode")
 )