fix(docs): correct retry_extended module path in waiter docs

why: Sphinx autodoc failed with ModuleNotFoundError
what:
- Change libtmux.test.retry_extended to libtmux._internal.retry_extended

Author: tony

docs/internals/waiter.md

@@ -127,7 +127,7 @@ All examples in this documentation are actual test files from the libtmux test s
 ## Extended Retry Functionality
 
 ```{eval-rst}
-.. automodule:: libtmux.test.retry_extended
+.. automodule:: libtmux._internal.retry_extended
    :members:
    :undoc-members:
    :show-inheritance: