[numpy] Duplicated {index} directive under Matrix Multiplication

numpy.md carries the same index directive twice, back to back, under the "Matrix Multiplication" heading (lines 432–438):

```{index} single: NumPy; Matrix Multiplication

One of the two should be removed.

Found during native-editor review of the French edition, where the duplication had been carried over by translation; the French copy was cleaned up in QuantEcon/lecture-python-programming.fr#24. Until it is removed here, the French edition deliberately diverges from the source at this spot. Same discovery path as #594.

numpy.md を開き、「Matrix Multiplication」見出しの下にある 432–438 行目を確認してください。連続する同一の {index} ディレクティブを 2 つのうち 1 つ削除し、その後、NumPy; Matrix Multiplication のエントリが 1 つだけ残っており、ソースが整理済みのフランス語版と一致することを確認してください。