Incorrect parameter ordering in element_sub

The parameter ordering of a call to element_wise within one of the element_sub overloads is incorrect.

Since the function signature of element_wise is:
```
void element_wise(ublas::matrix const &a,
ublas::matrix const &b,
ublas::matrix &result,
O op, compute::command_queue& queue)
```

The call in the first element_sub overload should be changed to:
`element_wise(a, b, result, compute::minus(), queue);`
as opposed to
`element_wise(a, b, compute::minus(), result, queue);`