message: docs(header): update ContentType example to use ContentType::json()
diff: @@ -27,12 +27,11 @@ header! {
  /// # Examples
  /// ```
  /// use hyper::header::{Headers, ContentType};
  - /// use hyper::mime::{Mime, TopLevel, SubLevel};
  ///
  /// let mut headers = Headers::new();
  ///
  /// headers.set(
  - /// ContentType(Mime(TopLevel::Text, SubLevel::Html, vec![]))
  + /// ContentType::json()
  /// );
  /// ```
  /// ```
  @@ -42,7 +41,7 @@ header! {
  /// let mut headers = Headers::new();
  ///
  /// headers.set(
  - /// ContentType(Mime(TopLevel::Application, SubLevel::Json,
  + /// ContentType(Mime(TopLevel::Text, SubLevel::Html,
  /// vec![(Attr::Charset, Value::Utf8)]))
  /// );
  /// ```
  