docs: update contributing re documentation gen
docs: update contributing re documentation gen

Documentation is no longer run from the gh-pages branch and is just using master. This reflects that change
mhenderson-so/bosun,bosun-monitor/bosun,bookingcom/bosun,bookingcom/bosun,bookingcom/bosun,bosun-monitor/bosun,bosun-monitor/bosun,bookingcom/bosun,mhenderson-so/bosun,bookingcom/bosun,mhenderson-so/bosun,bosun-monitor/bosun,mhenderson-so/bosun,bosun-monitor/bosun,bookingcom/bosun,mhenderson-so/bosun,bosun-monitor/bosun,mhenderson-so/bosun