typo fix in update.rst
typo fix in update.rst

thread-save -> thread-safe
bioinformed/pysam,pysam-developers/pysam,bioinformed/pysam,kyleabeauchamp/pysam,pysam-developers/pysam,TyberiusPrime/pysam,TyberiusPrime/pysam,kyleabeauchamp/pysam,bioinformed/pysam,TyberiusPrime/pysam,TyberiusPrime/pysam,TyberiusPrime/pysam,bioinformed/pysam,pysam-developers/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam,pysam-developers/pysam,bioinformed/pysam