Update usage.rst
Update usage.rst

Updating file name to .sam file. Usually my SAM files do not have .bam extensions. I think this makes a difference for readability.
brendanofallon/pysam,bioinformed/pysam,kyleabeauchamp/pysam,daler/pysam,kyleabeauchamp/pysam,nlhepler/pysam,pysam-developers/pysam,mattions/pysam,nlhepler/pysam,dannon/pysam,bioinformed/pysam,carolinux/pysam,mattions/pysam,pysam-developers/pysam,brendanofallon/pysam,mattions/pysam,kyleabeauchamp/pysam,carolinux/pysam,bioinformed/pysam,kyleabeauchamp/pysam,brendanofallon/pysam,dannon/pysam,daler/pysam,carolinux/pysam,bioinformed/pysam,daler/pysam,TyberiusPrime/pysam,carolinux/pysam,bioinformed/pysam,TyberiusPrime/pysam,daler/pysam,brendanofallon/pysam,nlhepler/pysam,pysam-developers/pysam,dannon/pysam,carolinux/pysam,mattions/pysam,TyberiusPrime/pysam,nlhepler/pysam,dannon/pysam,TyberiusPrime/pysam,mattions/pysam,daler/pysam,pysam-developers/pysam,dannon/pysam,nlhepler/pysam,TyberiusPrime/pysam,brendanofallon/pysam,kyleabeauchamp/pysam