Modifying Mac git install directions

The first line says to download git, but this only works for OSX10.8
and higher. This fix makes clear that this is only for OSC 10.8 and
encourages students to read the second line to find versions for 10.5
- 10.7
Changing wording at Raniere's suggestion
The language "use the most recent available" was confusing to me.
I added "for your OS"
Merge pull request #710 from abostroem/fix_git_setup_languages

Modifying Mac git install directions