docs: stop pinning a library version the instructions cannot keep current

Four places told people to write `soulseek-rs-lib = "8"` (RELEASE.md said "6"),
and every one of them had been left behind by ten majors or more. The failure is
silent rather than loud: 6.x and 8.0.0 are both real published versions, so cargo
resolves the snippet happily and hands over a library from August instead of
refusing it, and one of the four is soulseek-rs-lib's own README, which is the
page crates.io renders.

`cargo add` writes whatever version is current at the time it runs, so there is
no longer a number in the docs to go stale. The manifest pin in
soulseek-rs/Cargo.toml stays as it is — release-plz owns that one and keeps it
in step with the release.

soulseek-rs-lib/README.md

@@ -11,11 +11,10 @@ network. It can be used to build custom Soulseek clients.
 
 ## Usage
 
-Add this to your `Cargo.toml`:
+Add it to your `Cargo.toml`:
 
-```toml
-[dependencies]
-soulseek-rs-lib = "8"
+```bash
+cargo add soulseek-rs-lib
 ```
 
 ## Example