Remove edition2018 marker from example code
The whole crate is on 2018 edition now so these are ignored.
diff --git a/src/lib.rs b/src/lib.rs
index d7f659c..2ba3de2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -33,7 +33,7 @@
//!
//! The skeleton of a typical procedural macro typically looks like this:
//!
-//! ```edition2018
+//! ```
//! extern crate proc_macro;
//!
//! # const IGNORE: &str = stringify! {
@@ -768,7 +768,7 @@
/// A span must be provided explicitly which governs the name resolution
/// behavior of the resulting identifier.
///
-/// ```edition2018
+/// ```
/// use proc_macro2::{Ident, Span};
///
/// fn main() {
@@ -780,7 +780,7 @@
///
/// An ident can be interpolated into a token stream using the `quote!` macro.
///
-/// ```edition2018
+/// ```
/// use proc_macro2::{Ident, Span};
/// use quote::quote;
///
@@ -799,7 +799,7 @@
/// A string representation of the ident is available through the `to_string()`
/// method.
///
-/// ```edition2018
+/// ```
/// # use proc_macro2::{Ident, Span};
/// #
/// # let ident = Ident::new("another_identifier", Span::call_site());