Vim: change "desc" Jasmine snippet trigger to "des"

I found my fingers always wanting to type "des<Tab>", so I decided to
change the trigger for this snippet to match my fingers. This is less
typing anyway, which is what snippets are all about.
Vim: change final placeholders for Jasmine snippets

I found that when I had a `$2` here my tabbing was stuck in the snippet
tabstops longer than I expected.

The `$0` is a special tabstop that is always the last tabstop in the
snippet no matter how many tabstops are defined. Using this instead
seems to improve when UltiSnips knows when I am done with tabstops and
goes back to normal tabbing behavior.