Update docs to use Signed from num_traits instead of pyo3_polars

[chrisschopp]

In 2. How to do ABSolutely nothing, I was getting error:

error[E0433]: failed to resolve: could not find `export` in `polars_core`
 --> src\expressions.rs:2:39
  |
2 | use pyo3_polars::export::polars_core::export::num::Signed;
  |                                       ^^^^^^ could not find `export` in `polars_core`

Based on src/expressions.rs and Cargo.toml, I think Signed should be imported from num_traits.

[MarcoGorelli]

thanks @chrisschopp !