update docs to reflect cargo name `ssz_rs`

[ralexstokes]

Due to originally grabbing ssz_rs (instead of ssz-rs) on crates.io, this crate is stuck with the name.

We should make a pass on the docs etc to make sure things are easy for newcomers (i.e. Cargo.toml examples have ssz_rs)

A more drastic option would be to move this repo to something like simple-serialize and use the new name with hyphens but this would ripple out to users and ultimately be another source of confusion.