Fix type hint for client_credential

Author: jiasli

msal/application.py

@@ -120,7 +120,7 @@ def __init__(
 
         :param str client_id: Your app has a client_id after you register it on AAD.
 
-        :param str client_credential:
+        :param Union[str, dict] client_credential:
             For :class:`PublicClientApplication`, you simply use `None` here.
             For :class:`ConfidentialClientApplication`,
             it can be a string containing client secret,
@@ -1343,4 +1343,3 @@ def acquire_token_on_behalf_of(self, user_assertion, scopes, claims_challenge=No
             **kwargs))
         telemetry_context.update_telemetry(response)
         return response
-