chore: avoid Coe.coe in Data.Multiset.Functor

[Ruben-VandeVelde]

This was ported incorrectly from mathlib3.

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[github-actions]

PR summary ab08c7acc6

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/declarations_diff.sh <optional_commit>

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

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

[eric-wieser]

bors d+

Could also use Multiset.ofList which is a bit shorter, and what the notation amounts to.

[mathlib-bors]

✌️ Ruben-VandeVelde can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.