[tests] Update README after filename change

Author: conscott

test/README.md

@@ -33,13 +33,13 @@ The ZMQ functional test requires a python ZMQ library. To install it:
 Individual tests can be run by directly calling the test script, eg:
 
 ```
-test/functional/replace-by-fee.py
+test/functional/feature_rbf.py
 ```
 
 or can be run through the test_runner harness, eg:
 
 ```
-test/functional/test_runner.py replace-by-fee.py
+test/functional/test_runner.py feature_rbf.py
 ```
 
 You can run any combination (incl. duplicates) of tests by calling: