Fixed grammar and whitespace issues

[dczarnota]

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:

Now that we have a model that exist 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)

333:

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).

AFTER:
310-311

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).

333:

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).