(docs) fix broken links to Augeas wiki pages

According to
https://www.redhat.com/archives/augeas-devel/2014-November/msg00014.html
the pages in the Augeas wiki are to be accessed using URLs like
https://github.com/hercules-team/augeas/wiki/Name-of-the-wiki-page,
not http://augeas.net/page/Name_of_the_wiki_page. The links in
augeas.markdown, using the old URLs, became broken when the Augeas
team moved their wiki. This change fixes the links.