docs(dialog): update dialog afterClosed (#3411)

Author: Heo

src/lib/dialog/dialog.md

@@ -17,7 +17,7 @@ The `MdDialogRef` provides a handle on the opened dialog. It can be used to clos
 receive notification when the dialog has been closed.
 
 ```ts
-dialogRef.afterClosed.subscribe(result => {
+dialogRef.afterClosed().subscribe(result => {
   console.log(`Dialog result: ${result}`); // Pizza!
 });
 
@@ -76,4 +76,4 @@ that the AOT compiler knows to create the `ComponentFactory` for it.
   bootstrap: [AppComponent]
 })
 export class AppModule() {}
-```
+```