Update usage.rst
Update usage.rst

The code example involving the "Counter" class results in error on python3. This change would fix it.
nlhepler/pysam,kyleabeauchamp/pysam,carolinux/pysam,carolinux/pysam,daler/pysam,TyberiusPrime/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam,nlhepler/pysam,pysam-developers/pysam,pysam-developers/pysam,carolinux/pysam,mattions/pysam,nlhepler/pysam,dannon/pysam,daler/pysam,mattions/pysam,bioinformed/pysam,mattions/pysam,mattions/pysam,carolinux/pysam,daler/pysam,brendanofallon/pysam,carolinux/pysam,TyberiusPrime/pysam,dannon/pysam,dannon/pysam,pysam-developers/pysam,bioinformed/pysam,kyleabeauchamp/pysam,TyberiusPrime/pysam,mattions/pysam,TyberiusPrime/pysam,daler/pysam,nlhepler/pysam,daler/pysam,nlhepler/pysam,dannon/pysam,TyberiusPrime/pysam,pysam-developers/pysam,bioinformed/pysam,dannon/pysam,kyleabeauchamp/pysam,brendanofallon/pysam,brendanofallon/pysam,bioinformed/pysam,bioinformed/pysam,brendanofallon/pysam,brendanofallon/pysam