sync: fix typo in doc

Change-Id: Ie1f35c7598bd2549a048d64e1b1279bf4acaa103
GitHub-Last-Rev: c8cc7dfef987cbd04f48daabf23efa64c0c67322
GitHub-Pull-Request: golang/go#28051
Reviewed-on: https://go-review.googlesource.com/c/140302Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/sync/runtime.go

@@ -54,7 +54,7 @@ func init() {
 }
 
 // Active spinning runtime support.
-// runtime_canSpin returns true is spinning makes sense at the moment.
+// runtime_canSpin returns true if spinning makes sense at the moment.
 func runtime_canSpin(i int) bool
 
 // runtime_doSpin does active spinning.