refactor: update docstring for create_mtn_charge method

Modified the docstring of the create_mtn_charge method to simplify the description from "Create MTN MoMo charge" to "Create MTN charge" for clarity.

Author: princemuichkine

lomi/services/charges.py

@@ -20,7 +20,7 @@ def create_card_charge(self, body: Optional[Dict[str, Any]] = None) -> Any:
         return self._request("POST", path, data=body)
 
     def create_mtn_charge(self, body: Optional[Dict[str, Any]] = None) -> Any:
-        """Create MTN MoMo charge"""
+        """Create MTN charge"""
         path = "/charge/mtn"
         return self._request("POST", path, data=body)