message: docs(body): small wording improvement to body module docs
diff: //!
  //! There are two pieces to this in hyper:
  //!
  -//! - The [`HttpBody`](body::HttpBody) trait the describes all possible bodies.
  +//! - **The [`HttpBody`](body::HttpBody) trait** describes all possible bodies.
  //! hyper allows any body type that implements `HttpBody`, allowing
  //! applications to have fine-grained control over their streaming.
  -//! - The [`Body`](Body) concrete type, which is an implementation of
  +//! - **The [`Body`](Body) concrete type**, which is an implementation of
  //! `HttpBody`, and returned by hyper as a "receive stream" (so, for server
  //! requests and client responses). It is also a decent default implementation
  //! if you don't have very custom needs of your send streams.
  