Update CONTRIBUTING.md

Grammar fix.
I edited the file via the Github interface (as discussed in this file) and
it must have removed the new line.

On Sun, Oct 12, 2014 at 1:00 PM, James Smith <notifications@github.com>
wrote:

> In CONTRIBUTING.md:
>
> > \ No newline at end of file
> > +least get us started on creating the failing test in the code.
>
> What changed here?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/marionettejs/backbone.marionette/pull/1991/files#r18744990>
> .
>