MatrixMarket header is non-compliant

The MatrixMarket export of `nalgebra-sparse` produces the following header, with the lowercase word "matrixmarket":
```
%%matrixmarket matrix coordinate real general
```
While according to the [spec](https://math.nist.gov/MatrixMarket/formats.html#MMformat), it should be:
```
%%MatrixMarket matrix coordinate real general
```
This breaks reading MatrixMarket files in Python with scipy.io.mmread (tested with scipy-1.16.3).