fix sample config

this was apparently broken by #6040.

docs/sample_config.yaml

@@ -1520,7 +1520,7 @@ opentracing:
     #enabled: true
 
     # The list of homeservers we wish to send and receive span contexts and span baggage.
-    # See docs/opentracing.md
+    # See docs/opentracing.rst
     # This is a list of regexes which are matched against the server_name of the
     # homeserver.
     #