Fix docstring markup in sage/modular #34159
Description
Part of #34157:
sage/modular/overconvergent/hecke_series.py:295:1: RST303 Unknown directive type "todo".
sage/modular/overconvergent/genus0.py:163:1: RST399 Document may not end with a transition.
sage/modular/hecke/module.py:629:1: RST215 Inline interpreted text or phrase reference start-string without end-string.
sage/modular/pollack_stevens/space.py:665:1: RST399 Content block expected for the "WARNING" directive; none found.
sage/modular/pollack_stevens/sigma0.py:313:1: RST303 Unknown directive type "todo".
Component: documentation
Author: Frédéric Chapoton
Branch/Commit: 832de2e
Reviewer: Kwankyu Lee
Issue created by migration from https://trac.sagemath.org/ticket/34159