Update benchmarking/README.md

Co-authored-by: Adam Gleave <adam@gleave.me>

Author: timbauman

benchmarking/README.md

@@ -24,7 +24,7 @@ python -m imitation.scripts.analyze analyze_imitation with \
     run_name="<name>"
 ```
 
-To run a single benchmark from Python add the config to your experiment:
+To run a single benchmark from Python add the config to your Sacred experiment `ex`:
 
 ```python
 ...