sync: fix typo in documentation
R=rsc, adg
CC=golang-dev
https://golang.org/cl/2841041
diff --git a/src/pkg/sync/rwmutex.go b/src/pkg/sync/rwmutex.go
index 0058cf2..06fd0b0 100644
--- a/src/pkg/sync/rwmutex.go
+++ b/src/pkg/sync/rwmutex.go
@@ -21,7 +21,7 @@
// RLock locks rw for reading.
// If the lock is already locked for writing or there is a writer already waiting
-// to r the lock, RLock blocks until the writer has released the lock.
+// to release the lock, RLock blocks until the writer has released the lock.
func (rw *RWMutex) RLock() {
// Use rw.r.Lock() to block granting the RLock if a goroutine
// is waiting for its Lock. This is the prevent starvation of W in
