runtime/race: correct typo s/is/in

Change-Id: Ic79d97c2aa107c0e5c4a8906ad757b0390228bef
Reviewed-on: https://go-review.googlesource.com/c/go/+/210417Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/race/race.go

@@ -7,7 +7,7 @@
 package race
 
 // This file merely ensures that we link in runtime/cgo in race build,
-// this is turn ensures that runtime uses pthread_create to create threads.
+// this in turn ensures that runtime uses pthread_create to create threads.
 // The prebuilt race runtime lives in race_GOOS_GOARCH.syso.
 // Calls to the runtime are done directly from src/runtime/race.go.