in CONTRIBUTING, fix link to Acceptable Formulas

To match the recent renaming of [that wiki page](https://github.com/mxcl/homebrew/wiki/Acceptable-Formulas).

I renamed the wiki page from “Acceptable Formula” to “Acceptable Formulas” because I think it makes more sense. “This is an page about acceptable formulas [and what makes them acceptable].” If you don’t like the plural, you could name the page “An Acceptable Formula”, as in “an acceptable formula should have these properties”.