gc: fix typo.

There already is a "Phase 6" a few lines above.

R=rsc
CC=golang-dev
https://golang.org/cl/6270054

src/cmd/gc/lex.c

@@ -406,7 +406,7 @@ main(int argc, char *argv[])
 	if(nsavederrors+nerrors == 0)
 		fninit(xtop);
 
-	// Phase 6: Check external declarations.
+	// Phase 7: Check external declarations.
 	for(l=externdcl; l; l=l->next)
 		if(l->n->op == ONAME)
 			typecheck(&l->n, Erv);