sync: a couple of documentation nits.

R=golang-dev, agl1
CC=golang-dev
https://golang.org/cl/4079051

src/pkg/sync/mutex.go

@@ -5,7 +5,7 @@
 // The sync package provides basic synchronization primitives
 // such as mutual exclusion locks.  Other than the Once type,
 // most are intended for use by low-level library routines.
-// Higher-level synchronization  is better done via channels
+// Higher-level synchronization is better done via channels
 // and communication.
 package sync
 

src/pkg/sync/once.go

@@ -13,7 +13,7 @@ type Once struct {
 // Do calls the function f if and only if the method is being called for the
 // first time with this receiver.  In other words, given
 // 	var once Once
-// if Do(f) is called multiple times, only the first call will invoke f,
+// if once.Do(f) is called multiple times, only the first call will invoke f,
 // even if f has a different value in each invocation.  A new instance of
 // Once is required for each function to execute.
 //