go/ast: fix typo in comment

Change-Id: I719231466286b3c0135314388a7c560ef44d7c35
Reviewed-on: https://go-review.googlesource.com/20130
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/ast/commentmap.go

@@ -267,7 +267,7 @@ func (cmap CommentMap) Filter(node Node) CommentMap {
 }
 
 // Comments returns the list of comment groups in the comment map.
-// The result is sorted is source order.
+// The result is sorted in source order.
 //
 func (cmap CommentMap) Comments() []*CommentGroup {
 	list := make([]*CommentGroup, 0, len(cmap))