Fix ppa syntax in apt_repository module doc

The example for using a ppa had the wrong syntax.

Ironically, the correct syntax is illustrated in a comment on line 23.

library/apt_repository

@@ -48,7 +48,7 @@ notes:
    - A bug in C(apt-add-repository) always adds C(deb) and C(deb-src) types for repositories (see the issue on Launchpad U(https://bugs.launchpad.net/ubuntu/+source/software-properties/+bug/987264)), if a repo doesn't have source information (eg MongoDB repo from 10gen) the system will fail while updating repositories.
 author: Matt Wright
 examples:
-- code: "apt_repository: repo=ppa://nginx/stable"
+- code: "apt_repository: repo=ppa:nginx/stable"
   description: Add nginx stable repository from PPA
 - code: "apt_repository: repo='deb http://archive.canonical.com/ubuntu hardy partner'"
   description: Add specified repository into sources.