fix: 'this URL is not a hyperlink' error

[verpeteren]

When running cargo doc, an error is raised. This is resolved by following the compiler hint message:

error: this URL is not a hyperlink
 --> src/libm/mod.rs:3:26
  |
3 | //! This is pulled from  https://github.com/rust-lang-nursery/libm
  |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://github.com/rust-lang-nursery/libm>`
  |
  = note: bare URLs are not automatically turned into clickable links
note: the lint level is defined here
 --> src/libm/mod.rs:8:9
  |
8 | #![deny(warnings)]
  |         ^^^^^^^^
  = note: `#[deny(rustdoc::bare_urls)]` implied by `#[deny(warnings)]`

[arduano]

Closing this as libm will be a dependency in Cargo.toml again, via #53