ci: detect trailing whitespace

[Darksonn]

Trailing whitespace is generally always wrong. There are a few instances of it in the Tokio repository.

[Darksonn]

Confirmed that it works:

./tokio/src/net/unix/stream.rs:    ///                     println!("read {} bytes", n);        
./tokio/src/task/join_set.rs:    ///     }   
./tokio/src/task/join_set.rs:    ///     let output = set.join_all().await;  
./tokio/src/task/join_set.rs:    ///     }   
./tokio/src/task/join_set.rs:    ///     
./tokio/src/runtime/task_hooks.rs:/// **Note**: This is an [unstable API][unstable]. The public API of this type  
./tokio/src/runtime/task_hooks.rs:/// may break in 1.x releases. See [the documentation on unstable  
./tokio/src/runtime/task_hooks.rs:/// features][unstable] for details.  
./tokio/src/runtime/task_hooks.rs:///  
./tokio/src/runtime/task_hooks.rs:/// [unstable]: crate#unstable-features  
./tokio/src/runtime/builder.rs:    /// **Note**: This is an [unstable API][unstable]. The public API of this type  
./tokio/src/runtime/builder.rs:    /// may break in 1.x releases. See [the documentation on unstable  
./tokio/src/runtime/builder.rs:    /// features][unstable] for details.  
./tokio/src/runtime/builder.rs:    ///  
./tokio/src/runtime/builder.rs:    /// [unstable]: crate#unstable-features  
./tokio/src/runtime/builder.rs:    /// **Note**: This is an [unstable API][unstable]. The public API of this type  
./tokio/src/runtime/builder.rs:    /// may break in 1.x releases. See [the documentation on unstable  
./tokio/src/runtime/builder.rs:    /// features][unstable] for details.  
./tokio/src/runtime/builder.rs:    ///  
./tokio/src/runtime/builder.rs:    /// [unstable]: crate#unstable-features  
./CONTRIBUTING.md:run loom tests that test unstable features. 
./CONTRIBUTING.md:if the command rejects a word, you should backtick the rejected word if it's code related. If not, the 
./CONTRIBUTING.md:rejected word should be put into `spellcheck.dic` file. 
./.github/buildomat/README.md:illumos 
./.circleci/config.yml:version: 2.1        
./tokio-util/src/task/spawn_pinned.rs:///     
./tokio-util/src/task/spawn_pinned.rs:///         }