docs: fix Prometheus README snippet, remove nonexistent NewHandler()

Handler has never had a NewHandler constructor at any version (confirmed
back through v4.7.2) -- it's a plain struct, used via a literal or the
package's own DefaultHandler var. This snippet was added in 7e7070d
alongside the OTLP exporter and appears to be a copy-paste from
otlp.NewHandler (a real constructor on that subpackage). Anyone following
the README as written gets a compile error.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: kinghorn-twilio

README.md

@@ -197,11 +197,14 @@ import (
     "github.com/segmentio/stats/v5/prometheus"
 )
 
-handler := prometheus.NewHandler()
-stats.Register(handler)
-http.Handle("/metrics", handler)
+stats.Register(prometheus.DefaultHandler)
+http.Handle("/metrics", prometheus.DefaultHandler)
 ```
 
+`Handler` has no `NewHandler` constructor; `DefaultHandler` is a ready-to-use zero-config
+instance. Construct your own `&prometheus.Handler{}` literal instead if you need to set
+`TrimPrefix`, `MetricTimeout`, or `Buckets`.
+
 ### InfluxDB
 
 The [github.com/segmentio/stats/v5/influxdb](https://godoc.org/github.com/segmentio/stats/v5/influxdb) package sends metrics to InfluxDB using the line protocol over HTTP.