Update coverage.rst

I noticed that http://bedtools.readthedocs.org/en/latest/content/tools/coverage.html does not correctly display the examples (i.e. the line breaks within all sections marked with :: are ignored). Here, on github, it looks like it should look. I have no experience whatsoever with ReST, but based on a quick google search I thought that replacing the :: by .. code:: might help. As github displays the snippets correctly with both versions I do not know whether it makes a difference for the external website's display. Best, Friederike