fix various comments

(os package comments were duplicates)

R=rsc
CC=golang-dev
https://golang.org/cl/2205042

src/pkg/go/doc/comment.go

@@ -194,7 +194,7 @@ var (
 // into a link). Go identifiers that appear in the words map are italicized; if
 // the corresponding map value is not the empty string, it is considered a URL
 // and the word is converted into a link. If nice is set, the remaining text's
-// appearance is improved where is makes sense (e.g., `` is turned into “
+// appearance is improved where it makes sense (e.g., `` is turned into “
 // and '' into ”).
 func emphasize(w io.Writer, line []byte, words map[string]string, nice bool) {
 	for {

src/pkg/os/file_unix.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// The os package provides a platform-independent interface to operating
-// system functionality.  The design is Unix-like.
 package os
 
 import (

src/pkg/os/file_windows.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// The os package provides a platform-independent interface to operating
-// system functionality.  The design is Unix-like.
 package os
 
 import (