bpo-29862: Fix grammar in importlib.reload() exception (GH-809)

Lib/importlib/__init__.py

@@ -137,7 +137,7 @@ def reload(module):
 
     """
     if not module or not isinstance(module, types.ModuleType):
-        raise TypeError("reload() argument must be module")
+        raise TypeError("reload() argument must be a module")
     try:
         name = module.__spec__.name
     except AttributeError: