Remove `buckets` from Telemetry guide
Remove `buckets` from Telemetry guide

This config option is reporter specific as of telemetry_metrics 0.5.0
and can be provided with `reporter_options: [buckets: [100, 200, 300]]`

Mentioning that here is outside the scope of the guide as the reporter
will warn if it is missing a required configuration option.

0x00evil/phoenix,nshafer/phoenix,rafbgarcia/phoenix,0x00evil/phoenix,optikfluffel/phoenix,phoenixframework/phoenix,phoenixframework/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,nshafer/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,optikfluffel/phoenix,nshafer/phoenix,0x00evil/phoenix