remove references and docs about using subversion to download LAMMPS

doc/src/Howto_github.rst

@@ -12,7 +12,7 @@ changes or additions you have made to LAMMPS into the official LAMMPS
 distribution.  It uses the process of updating this very tutorial as
 an example to describe the individual steps and options.  You need to
 be familiar with git and you may want to have a look at the
-`Git book <http://git-scm.com/book/>`_ to reacquaint yourself with some
+`git book <http://git-scm.com/book/>`_ to reacquaint yourself with some
 of the more advanced git features used below.
 
 As of fall 2016, submitting contributions to LAMMPS via pull requests