reword installation docs re: libc++ vs libstdc++
reword installation docs re: libc++ vs libstdc++

ndawe/root_numpy,rootpy/root_numpy,rootpy/root_numpy,ndawe/root_numpy,rootpy/root_numpy,scikit-hep/root_numpy,rootpy/root_numpy,scikit-hep/root_numpy,scikit-hep/root_numpy,ndawe/root_numpy,scikit-hep/root_numpy,ndawe/root_numpy