runtime: update comment to match function name

Change-Id: I8f22434ade576cc7e3e6d9f357bba12c1296e3d1
Reviewed-on: https://go-review.googlesource.com/15250Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/signal1_unix.go

@@ -195,7 +195,7 @@ func crash() {
 	raise(_SIGABRT)
 }
 
-// createSigM starts one global, sleeping thread to make sure at least one thread
+// ensureSigM starts one global, sleeping thread to make sure at least one thread
 // is available to catch signals enabled for os/signal.
 func ensureSigM() {
 	if maskUpdatedChan != nil {