go/ast: fix typo in comment

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/111330043

src/pkg/go/ast/scope.go

@@ -80,7 +80,7 @@ type Object struct {
 	Name string      // declared name
 	Decl interface{} // corresponding Field, XxxSpec, FuncDecl, LabeledStmt, AssignStmt, Scope; or nil
 	Data interface{} // object-specific data; or nil
-	Type interface{} // place holder for type information; may be nil
+	Type interface{} // placeholder for type information; may be nil
 }
 
 // NewObj creates a new object of a given kind and name.