Fix typo

substra_template/run_compute_plan.py

@@ -9,7 +9,7 @@ from dotenv import load_dotenv
 import os
 load_dotenv()
 
-NUM_CLIENTS = int(os.environ["NUM_CLIENTS"])
+NUM_CLIENTS = int(os.environ["SUBSTRA_NUM_HOSPITALS"])
 
 seed = 42
 torch.manual_seed(seed)