Docs: Omit suffix for bed_file

[wwgordon]

In the documentation for king.py, config parameters 2 and 3 are gds_file and bed_file. gds_file requires the full file name, including the ".gds" suffix, while bed_file requires the filename with the suffix omitted since multiple suffixes (.bed, .bim, .fam., etc.) will be added. It would be helpful if it was explicitly stated that bed_file should not include a suffix.