Fixed a typo where "exist" should be "exists"

Exists in lines 222 and 248 matches the singular nouns it is being used with. "Exist" had a mismatch where it expected plural nouns.

Author: ClimbsRocks

_posts/2011-01-29-what-is-a-model.md

@@ -219,7 +219,7 @@ If we instantiate a model with an `id`, Backbone.js will automatically perform a
 
 ### Updating a model
 
-Now that we have a model that exist on the server we can perform an update using a PUT request.
+Now that we have a model that exists on the server we can perform an update using a PUT request.
 We will use the `save` api call which is intelligent and will send a PUT request instead of a POST request if an `id` is present(conforming to RESTful conventions)
 
 {% highlight javascript %}
@@ -245,7 +245,7 @@ We will use the `save` api call which is intelligent and will send a PUT request
 
 ### Deleting a model
 
-When a model has an `id` we know that it exist on the server, so if we wish to remove it from the server we can call `destroy`.  `destroy` will fire off a DELETE /user/id (conforming to RESTful conventions).
+When a model has an `id` we know that it exists on the server, so if we wish to remove it from the server we can call `destroy`.  `destroy` will fire off a DELETE /user/id (conforming to RESTful conventions).
 
 {% highlight javascript %}