runtime: fix typo in comment

Change-Id: I96db053184e5e72864514d5421a97774545cc2dd
GitHub-Last-Rev: f1451ab626563f82f1703a559e4cb6d66665a7b6
GitHub-Pull-Request: golang/go#36425
Reviewed-on: https://go-review.googlesource.com/c/go/+/213597Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os_linux.go

@@ -343,7 +343,7 @@ func minit() {
 	minitSignals()
 
 	// Cgo-created threads and the bootstrap m are missing a
-	// procid. We need this for asynchronous preemption and its
+	// procid. We need this for asynchronous preemption and it's
 	// useful in debuggers.
 	getg().m.procid = uint64(gettid())
 }