Fix typo in first-app-tests.jade

Author: m5rk

public/docs/ts/latest/testing/first-app-tests.jade

@@ -185,7 +185,7 @@ figure.image-display
   Once configured with a default extension of 'js',   SystemJS requests `hero.js` which *does* exist and is promptly returned by our server.
 
   ### Asynchronous System.import
-  The call to `System.import` shouldn't surprise us but it's asynchronous nature might.
+  The call to `System.import` shouldn't surprise us but its asynchronous nature might.
   If we ponder this for a moment, we realize that it must be asynchronous because
   System.js may have to fetch the corresponding JavaScript file from the server.
   Accordingly, `System.import` returns a promise and we must wait for that promise to resolve.