Modifying Mac git install directions

The first line of the Mac git directions says to download git, but this only works for OSX 10.8
and higher. This fix makes clear that this is only for OSX 10.8 and
encourages students to read the second line to find the correct download for 
versions for 10.5 - 10.7

My editor automatically strips trailing whitespace, that is the change in line 247.