Remove mention of non-existent method.
// (https://godoc.org/github.com/sirupsen/logrus#Logger.WithFields) is
// supported. However, if the field key collides with one of the keys used by
// the New Relic Formatter, the value will be overwritten. Reserved keys are
-// those returned from `txn.GetLinkingMetadata().Map()`
-// (https://godoc.org/github.com/newrelic/go-agent/#LinkingMetadata.Map) and
// those found in the `logcontext` package
// (https://godoc.org/github.com/newrelic/go-agent/_integrations/logcontext/#pkg-constants).
//
