fix README.md poisson command

Author: christopher-wild

examples/poisson/README.md

@@ -6,6 +6,6 @@ To run this case study:
 1. Ensure all project dependencies are installed by running `pip install .` in the top level directory
    (instructions are provided in the project README).
 2. Change directory to `causal_testing/examples/poisson`.
-3. Run the command `python test_run_causal_tests.py --data_path data.csv --dag_path dag.dot --json_path causal_tests.json`
+3. Run the command `python example_run_causal_tests.py --data_path data.csv --dag_path dag.dot --json_path causal_tests.json`
 
 This should print a series of causal test results and produce two CSV files. `intensity_num_shapes_results_random_1000.csv` corresponds to table 1, and `width_num_shapes_results_random_1000.csv` relates to our findings regarding the relationship of width and `P_u`.