example: fix call to `jl_apply_array_type` in multidimensional array example for embedding (#44801)
Without an explicit cast I get an error:
```
fast_cholesky.cpp:9:50: error: cannot convert ‘jl_datatype_t*’ {aka ‘_jl_datatype_t*’} to ‘jl_value_t*’ {aka ‘_jl_value_t*’}
9 | jl_value_t *array_type = jl_apply_array_type(jl_float64_type, 2);
| ^~~~~~~~~~~~~~~
| |
| jl_datatype_t* {aka _jl_datatype_t*}
In file included from fast_cholesky.cpp:1:
/usr/include/julia/julia.h:1533:58: note: initializing argument 1 of ‘jl_value_t* jl_apply_array_type(jl_value_t*, size_t)’
1533 | JL_DLLEXPORT jl_value_t *jl_apply_array_type(jl_value_t *type, size_t dim);
| ~~~~~~~~~~~~^~~~
```