Update DMS "Translation Center" link: from 404 to Transifex

From 404 to the DMS Transifex translation project.

Ref. https://github.com/pagelines/DMS/tree/1.1/language/readme.txt

	PageLines I18n
	==============
	
	Join our translation task force at
	www.transifex.com/projects/p/pagelines-dms/