Fix typo.

According to an issue I posted here: https://github.com/SirVer/ultisnips/issues/338 sec.snippets is plural and should be singular.  I do realize that the bug report is against a different tool than snipmate, but it only stands to reason that since all of the other files in that directory are singular, this one probably should be as well.  Doing so also fixes the error I'm seeing.