Fix link to Leipzig corpus

Fix the link to the Leipzig corpus in the documentation:

The Leipzig Corpora collection presents corpora in different languages. The corpora is a collection of individual sentences collected from the web and newspapers. The Corpora is available as plain text and as MySQL database tables. The OpenNLP integration can only use the plain text version. The individual plain text packages can be downloaded here: http://corpora.uni-leipzig.de/download.html