Fix document source link (#514)
Fix document source link (#514)

PR-URL: #514
matthewloring/cloud-trace-nodejs,matthewloring/cloud-trace-nodejs,googleapis/cloud-trace-nodejs,googleapis/cloud-trace-nodejs,keymetrics/vxx,googleapis/cloud-trace-nodejs,matthewloring/cloud-trace-nodejs,googleapis/cloud-trace-nodejs,keymetrics/vxx,keymetrics/trassingue,keymetrics/trassingue