change openmm url

openmm.org seems to be down, no idea whether this persists.
Github page should be equally good...

doc/buildsystem.rst

@@ -165,7 +165,7 @@ safely delete the whole source folder.
 .. _python: https://www.python.org/
 .. _boost: https://www.boost.org/
 .. _eigen3: http://eigen.tuxfamily.org/index.php?title=Main_Page
-.. _openmm: http://openmm.org
+.. _openmm: https://github.com/openmm/openmm
 
 ..  LocalWords:  cmake makefiles CMakeLists txt DOCTEST LINKCHECK conf html
 ..  LocalWords:  doctest linkcheck qmean DQMEAN eigen CMakeCache