docs(toh-4): reword the getHeroesSlowly instructions

closes #1011
rewording to: "replace `_heroService.getHeroes` with `_heroService.getHeroesSlowly`"

Author: JamesLaverack

public/docs/ts/latest/tutorial/toh-pt4.jade

@@ -384,7 +384,7 @@ code-example(format="." language="html").
   Like `getHeroes`, it also returns a promise. 
   But this promise waits 2 seconds before resolving the promise with mock heroes.
 
-  Back in the `AppComponent`, swap `_heroService.getHeroesSlowly` for `_heroService.getHeroes`
+  Back in the `AppComponent`, replace `_heroService.getHeroes` with `_heroService.getHeroesSlowly`
   and see how the app behaves.
 
 .l-main-section