Remove doc URL from Cargo.toml

https://doc.rust-lang.org/cargo/reference/manifest.html#the-documentation-field

> If no URL is specified in the manifest file, crates.io will
> automatically link your crate to the corresponding docs.rs page.

Cargo.toml

@@ -7,6 +7,5 @@ description = "Run closures in parallel"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/smol-rs/easy-parallel"
 homepage = "https://github.com/smol-rs/easy-parallel"
-documentation = "https://docs.rs/easy-parallel"
 keywords = ["scope", "thread", "scoped", "spawn"]
 categories = ["concurrency"]