Fixing output directory for documentation

In order to make the css work I've changed in a previous commit the working directory for doxygen to CMAKE_CURRENT_SOURCE_DIR; the output is placed in CMAKE_CURRENT_BINARY_DIR, and that changed as a result of my change.

This caused the http://buildbot.opencog.org/doxygen/index.html page to be forbidden even if the build succeded. My bad, then.

Long story short, the output is now back where it should be.