Fix a typo and add a warning on how long git svn fetch can take

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2128 ed100bfc-0583-0410-97f2-c26b58777a21
Fixes #168

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2129 ed100bfc-0583-0410-97f2-c26b58777a21