get rid of log.NewContext mention in log/level doc

log/level/doc.go

@@ -5,7 +5,7 @@
 //    var logger log.Logger
 //    logger = log.NewLogfmtLogger(os.Stderr)
 //    logger = level.NewFilter(logger, level.AllowInfoAndAbove()) // <--
-//    logger = log.NewContext(logger).With("ts", log.DefaultTimestampUTC)
+//    logger = log.With(logger, "ts", log.DefaultTimestampUTC)
 //
 // Then, at the callsites, use one of the level.Debug, Info, Warn, or Error
 // helper methods to emit leveled log events.