Minor fix to dir-spec.txt
I just found that dir-spec.txt refers to keyword "geoip-start-time" as "geoip-start". This keyword was replaced by "bridge-stats-end" in commit e1e5c1b3aba96afbd6fa3b5ece262720dd45a80e. Should we update dir-spec.txt anyway to say "geoip-start-time"? Should I make a patch?