CONTRIBUTING: fix link to Acceptable Formulae.

To match the recent renaming of that wiki article.

Closes Homebrew/homebrew#17249.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

CONTRIBUTING.md

@@ -15,6 +15,6 @@ Contributing
 Please read:
 
 * [Formula Cookbook](https://github.com/mxcl/homebrew/wiki/Formula-Cookbook)
-* [Acceptable Formula](https://github.com/mxcl/homebrew/wiki/Acceptable-Formula)
+* [Acceptable Formulas](https://github.com/mxcl/homebrew/wiki/Acceptable-Formulas)
 
 Thanks!