Description of slice is misleading#124

The chapter about slices https://github.com/nrc/r4cppp/blob/master/arrays.md#slices uses wrong comparisons. It states > A slice in Rust is just an array whose length is not known at compile time. But the rust documentation about slices says https://doc.rust-lang.org/std/slice/ > A dynamically-sized view into a contiguous sequence, [T]. So a rust `slice` is similar to a C++ `std:span` https://en.cppreference.com/w/cpp/container/span and not similar to a rust `array` (a value) or to an C/C++ `array` (only a pointer).