go/ast: fixed minor typo in comment

Fixes #10125.

Change-Id: I99032912650cef8e5bf124bda0a60d5899760103
Reviewed-on: https://go-review.googlesource.com/7320Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/ast/scope.go

@@ -38,7 +38,7 @@ func (s *Scope) Lookup(name string) *Object {
 // Insert attempts to insert a named object obj into the scope s.
 // If the scope already contains an object alt with the same name,
 // Insert leaves the scope unchanged and returns alt. Otherwise
-// it inserts obj and returns nil."
+// it inserts obj and returns nil.
 //
 func (s *Scope) Insert(obj *Object) (alt *Object) {
 	if alt = s.Objects[obj.Name]; alt == nil {