docs(resource): fix grammatical issue

'Case' should be the plural 'cases' since it is talking about multiple possible cases rather
than a single case. For slightly more info, see the section 'When words like "none" are the
subject' in this article: http://writing.wisc.edu/Handbook/SubjectVerb.html

Author: mkolodny

src/ngResource/resource.js

@@ -151,7 +151,7 @@ function lookupDottedPath(obj, path) {
  *   usually the resource is assigned to a model which is then rendered by the view. Having an empty
  *   object results in no rendering, once the data arrives from the server then the object is
  *   populated with the data and the view automatically re-renders itself showing the new data. This
- *   means that in most case one never has to write a callback function for the action methods.
+ *   means that in most cases one never has to write a callback function for the action methods.
  *
  *   The action methods on the class object or instance object can be invoked with the following
  *   parameters: