fix docstring in get_subscription_summary client function (#1100)

* fix doc string

Author: charcey

Diff for: planet/clients/subscriptions.py

@@ -438,7 +438,7 @@ async def get_subscription_summary(
     ) -> dict:
         """Summarize the status of results for a single subscription via GET.
 
-        Args
+        Parameters:
             subscription_id (str): ID of the subscription to summarize.
 
         Returns: