Fixed url

doc/buildsystem.rst

@@ -106,7 +106,7 @@ special targets:
 .. _cmake: http://www.cmake.org/
 .. _python: https://www.python.org/
 .. _boost: http://www.boost.org/
-.. _eigen3: http://eigen.tuxfamily.org/
+.. _eigen3: http://eigen.tuxfamily.org/index.php?title=Main_Page
 .. _lapack: http://www.netlib.org/lapack/
 
 ..  LocalWords:  cmake makefiles CMakeLists txt DOCTEST LINKCHECK conf html