gc: correct comment in runtime.go

R=rsc, iant
CC=golang-dev
https://golang.org/cl/5777057

src/cmd/gc/runtime.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // NOTE: If you change this file you must run "./mkbuiltin"
-// to update builtin.c.boot.  This is not done automatically
+// to update builtin.c.  This is not done automatically
 // to avoid depending on having a working compiler binary.
 
 // +build ignore