Quick bugfix to bls_printer.py

Fixed an issue where extra '</tr>'s were being added to the end of the
header row.  One extra '</tr>' was being added for each month in the
year for which there was no BLS data.

Concrete example: in September 2013, BLS has no data for September,
October, November, or December yet -- so four extra '</tr>'s were being
mistakenly added.