gh-38215: Fix typo "sage.ring" in docstrings
The correct module path is "sage.rings" instead of "sage.ring".
In sage/modular/pollack_stevens/padic_lseries.py and in
sage/structure/factory.pyx the typo prevents doctests from being run
because the "needs" requirement canot be fulfilled.
In sage/rings/polynomial/padics/polynomial_padic.py the typo is merely
cosmetic.
See brief [discussion in Sage Support
Group](https://groups.google.com/g/sage-support/c/Msx0QJF7rpA)
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URL: https://github.com/sagemath/sage/pull/38215
Reported by: Jens-Erik Riedel
Reviewer(s): David Coudert, Julian Rüth
