Fix command for StartExternalSegment

[abeltay]

Details

There is no command for txn.StartExternalSegment for both v3 version and non-v3

go-agent/segments.go

Line 185 in 5a0eaeb

func StartExternalSegment(txn Transaction, request *http.Request) *ExternalSegment {

go-agent/v3/newrelic/segments.go

Line 282 in 5a0eaeb

func StartExternalSegment(txn *Transaction, request *http.Request) *ExternalSegment {

This PR updates the documentation to match the actual command provided by the package.

[CLAassistant]

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