Fix note about trace endpoint URL

Author: mpetazzoni

README.rst

@@ -50,7 +50,7 @@ Environment Variables
     SIGNALFX_TRACING_URL=tracing endpoint [ default: https://ingest.signalfx.com/v1/trace ]
 
 ``SIGNALFX_ENDPOINT_URL`` can be used to configure a common endpoint for metrics and
-traces, as is the case when forwarding with the Smart Gateway. The path ``/v1/traces``
+traces, as is the case when forwarding with the Smart Gateway. The path ``/v1/trace``
 will automatically be added to the endpoint for traces.
 
 If either ``SIGNALFX_TRACING_URL`` or ``SIGNALFX_METRICS_URL`` are set, they will take