chore: fix typo and add to simp

[xgenereux]

There is a typo in the first lemma.

As for the simp set, we of course can't have both, I think it makes more sense like this. I had an instance where having symm_toRingEquiv in the simp set would stop my simp call.

In particular when you are applying the morphism to an element you don't get the simplification via AlgEquiv.coe_ringEquiv.

(If we're given (↑f).symm a if should simplify to f.symm a which, to my understanding, is achieved by pulling out the coe and then applying AlgEquiv.coe_ringEquiv.)

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[github-actions]

PR summary b0a8bdbc5e

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).