runtime: fix missing word in comment

Change-Id: I6cb8ac7b59812e82111ab3b0f8303ab8194a5129
Reviewed-on: https://go-review.googlesource.com/19791Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/proc.go

@@ -2406,7 +2406,7 @@ func entersyscallblock_handoff() {
 // The goroutine g exited its system call.
 // Arrange for it to run on a cpu again.
 // This is called only from the go syscall library, not
-// from the low-level system calls used by the
+// from the low-level system calls used by the runtime.
 //go:nosplit
 func exitsyscall(dummy int32) {
 	_g_ := getg()