bpo-39968: Fix a typo error in get_readline_state() (GH-19028)

diff --git a/Modules/readline.c b/Modules/readline.c
index 8a1a2a8..225d06b 100644
--- a/Modules/readline.c
+++ b/Modules/readline.c
@@ -87,7 +87,7 @@
 } readlinestate;
 
 static inline readlinestate*
-get_readline_state(PyModule *module)
+get_readline_state(PyObject *module)
 {
     void *state = PyModule_GetState(module);
     assert(state != NULL);