Update link to "XBase File Format Description" 
Update link to "XBase File Format Description" 

Updated link to "XBase File Format Description"  to address Issue #12: Documentation URL not valid #12.  Link now properly links.
jzmiller1/pyshp-1,petabytekr/pyshp,GeospatialPython/pyshp,micahcochran/pyshp,CalebM1987/pyshp