Add optional page title for articles index page

The default page title for the articles index page is "Article listings". I have a need to change this title to the text entered in the "Overview Title" settings in administer/settings/article. Attached is a patch that will change the page title for the articles index page if "Overview Title" has an entry. If nothing has been saved in the "Overview Title" field, the page title will remain as "Article listings".

I also made a small spelling correction in the module's comments.

This module is great. It was just what I needed for our IT knowledgebase at http://ucit01.uncc.edu/knowledgebase.

Thanks!
Mark Shropshire
UCIT Computer System Administrator
UNC Charlotte