Remove fn main from doctest (and also remove edition2018 cause I think it isn't useful)

Fix clippy "warning: needless `fn main` in doctest"

src/types/inline_keyboard_button.rs

@@ -44,15 +44,13 @@ pub enum InlineKeyboardButtonKind {
 /// Build buttons
 ///
 /// Example:
-/// ```edition2018
+/// ```
 /// use async_telegram_bot::types::InlineKeyboardButton;
 ///
-/// fn main() {
 /// let url_button = InlineKeyboardButton::url(
 ///     "Text".to_string(),
 ///     "http://url.com".to_string(),
 /// );
-/// }
 /// ```
 impl InlineKeyboardButton {
     pub fn url(text: String, url: String) -> InlineKeyboardButton {