Fix typo in the coq-mathcomp-dioid compatibility shim - #2

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Use of "Notation" keyword for abbreviations is deprecated, use

Notations "_ ^+ _" defined at level 29 with arguments constr

Postfix notations (i.e. starting with a nonterminal symbol and