docs(tutorial/step-12): replaced a missing apostrophe and fixed gramm…

…ar errors

Fixed a missing apostrophe and some grammar in the Animating 'ngClass' with JavaScript section

docs/content/tutorial/step_12.ngdoc

@@ -293,8 +293,7 @@ Let's add another animation to our application. Switching to our `phone-detail.h
 we see that we have a nice thumbnail swapper. By clicking on the thumbnails listed on the page,
 the profile phone image changes. But how can we change this around to add animations?
 
-Lets think about it first,
-basically when you click on a thumbnail image, you're changing the state of the profile image to reflect the newly
+Let's think about it first. Basically, when you click on a thumbnail image, you're changing the state of the profile image to reflect the newly
 selected thumbnail image.
 The best way to specify state changes within HTML is to use classes.
 Much like before, how we used a CSS class to specify