gh-117961: Fix matmul grammar in expression docs

[imsofial]

Update the documented grammar for the @ operator so that it matches the associativity specified in PEP 465.

The current documentation shows m_expr "@" m_expr, which makes the associativity unclear. Since @ should parse like *, the right-hand side should be u_expr.

• Issue: Associativity of matmul unclear from grammar for m_expr  #117961

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[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #32509347 | 📁 Comparing 3ab4209 against main (efcac6f)

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2 files changed

± reference/expressions.html
± whatsnew/changelog.html

[StanFromIreland]

Please revert this change, this file is updated during release.