Update to the release instructions for undoing a botched release

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2387 ed100bfc-0583-0410-97f2-c26b58777a21
Add .0 to the tag if the version is just 3.Y (at some point I started an inadvertent convetion that the version number in setup.cfg omits the .0, but the tag name includes it)

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2388 ed100bfc-0583-0410-97f2-c26b58777a21