Fix `cargo doc` in nightly (#21)
Fix `cargo doc` in nightly (#21)

```
warning: `[spawn]` cannot be resolved, ignoring it...
  --> src/scoped.rs:16:28
   |
16 | /// [`std::thread::spawn`][spawn]:
   |                            ^^^^^ cannot be resolved, ignoring
   |
   = note: #[warn(intra_doc_link_resolution_failure)] on by default
   = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

warning: `[spawn]` cannot be resolved, ignoring it...
  --> src/scoped.rs:16:28
   |
16 | /// [`std::thread::spawn`][spawn]:
   |                            ^^^^^ cannot be resolved, ignoring
   |
   = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

warning: `[spawn]` cannot be resolved, ignoring it...
  --> src/scoped.rs:61:36
   |
61 | /// Because [`std::thread::spawn`][spawn] doesn't know about this scope, it requires a
   |                                    ^^^^^ cannot be resolved, ignoring
   |
   = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

```

$ rustc --version
rustc 1.28.0-nightly (01cc982e9 2018-06-24)
crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,aturon/crossbeam,crossbeam-rs/crossbeam