Fixed grammar and whitespace issues

Changed "exist" to "exists" on lines 310 and 333. At line 311, added whitespace between "present" and "(conforming to RESTful conventions)" as well as a period at the end of the sentence.

BEFORE:
310-311: <p>Now that we have a model that exist on the server we can perform an update using a PUT request.
We will use the <code>save</code> api call which is intelligent and will send a PUT request instead of a POST request if an <code>id</code> is present(conforming to RESTful conventions)</p>

333: <p>When a model has an <code>id</code> we know that it exist on the server, so if we wish to remove it from the server we can call <code>destroy</code>.  <code>destroy</code> will fire off a DELETE /user/id (conforming to RESTful conventions).</p>

AFTER:
310-311 <p>Now that we have a model that exists on the server we can perform an update using a PUT request.
We will use the <code>save</code> api call which is intelligent and will send a PUT request instead of a POST request if an <code>id</code> is present (conforming to RESTful conventions).</p>

333: <p>When a model has an <code>id</code> we know that it exists on the server, so if we wish to remove it from the server we can call <code>destroy</code>.  <code>destroy</code> will fire off a DELETE /user/id (conforming to RESTful conventions).</p>