cmd/gc: fix example flag in comment

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/118480044

src/cmd/gc/lex.c

@@ -45,7 +45,7 @@ static struct {
 };
 
 // Debug arguments.
-// These can be specified with the -d flag, as in "-d checknil"
+// These can be specified with the -d flag, as in "-d nil"
 // to set the debug_checknil variable. In general the list passed
 // to -d can be comma-separated.
 static struct {