api docs: Use method from our Python bindings in the example.

One of the code examples for GET /users was using the raw call_endpoint
method from our Python bindings, rather than get_members, which has been
specifically designed to interact with this endpoint.

Now all the examples here use the appropiate method.

zerver/lib/api_test_helpers.py

@@ -140,10 +140,7 @@ def get_members(client):
 
     # {code_example|start}
     # You may pass the `client_gravatar` query parameter as follows:
-    result = client.call_endpoint(
-        url='users?client_gravatar=true',
-        method='GET',
-    )
+    result = client.get_members({'client_gravatar': True})
     # {code_example|end}
 
     validate_against_openapi_schema(result, '/users', 'get', '200')