gc: fix stray %#N in error message

Fixes #2639.

R=rsc
CC=bradfitz, golang-dev
https://golang.org/cl/5489140

src/cmd/gc/typecheck.c

@@ -1013,7 +1013,7 @@ reswitch:
 			goto error;
 		}
 		if(!(t->chan & Csend)) {
-			yyerror("invalid operation: %#N (cannot close receive-only channel)", n);
+			yyerror("invalid operation: %N (cannot close receive-only channel)", n);
 			goto error;
 		}
 		ok |= Etop;