docs: update Python modules section of the guide (#3924)

guide/src/module.md

@@ -65,22 +65,22 @@ print(my_extension.__doc__)
 ## Python submodules
 
 You can create a module hierarchy within a single extension module by using
-[`PyModule.add_submodule()`]({{#PYO3_DOCS_URL}}/pyo3/prelude/struct.PyModule.html#method.add_submodule).
+[`Bound<'_, PyModule>::add_submodule()`]({{#PYO3_DOCS_URL}}/pyo3/prelude/trait.PyModuleMethods.html#tymethod.add_submodule).
 For example, you could define the modules `parent_module` and `parent_module.child_module`.
 
 ```rust
 use pyo3::prelude::*;
 
 #[pymodule]
-fn parent_module(py: Python<'_>, m: &PyModule) -> PyResult<()> {
-    register_child_module(py, m)?;
+fn parent_module(m: &Bound<'_, PyModule>) -> PyResult<()> {
+    register_child_module(m)?;
     Ok(())
 }
 
-fn register_child_module(py: Python<'_>, parent_module: &PyModule) -> PyResult<()> {
-    let child_module = PyModule::new_bound(py, "child_module")?;
+fn register_child_module(parent_module: &Bound<'_, PyModule>) -> PyResult<()> {
+    let child_module = PyModule::new_bound(parent_module.py(), "child_module")?;
     child_module.add_function(wrap_pyfunction!(func, &child_module)?)?;
-    parent_module.add_submodule(child_module.as_gil_ref())?;
+    parent_module.add_submodule(&child_module)?;
     Ok(())
 }