Change no_std to no-std in Cargo.toml (#991)

Otherwise I get the following error when attempting to publish the crate:
```
error: failed to publish to registry at https://crates.io

Caused by:
  the remote server responded with an error (status 400 Bad Request): The following category slugs are not currently supported on crates.io: no_std

  See https://crates.io/category_slugs for a list of supported slugs.
```

Signed-off-by: Valentin Gosu <1454649+valenting@users.noreply.github.com>

url/Cargo.toml

@@ -10,7 +10,7 @@ documentation = "https://docs.rs/url"
 repository = "https://github.com/servo/rust-url"
 readme = "../README.md"
 keywords = ["url", "parser"]
-categories = ["parser-implementations", "web-programming", "encoding", "no_std"]
+categories = ["parser-implementations", "web-programming", "encoding", "no-std"]
 license = "MIT OR Apache-2.0"
 include = ["src/**/*", "LICENSE-*", "README.md", "tests/**"]
 edition = "2018"