Change log prefix slightly
Change log prefix slightly

There is already a colon in the default log formatting, so elide it
from the prefix.

Signed-off-by: Daniel Farina <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@heroku.com>

heroku/pg_logplexcollector,logplex/pg_logplexcollector