Set all documentation examples to 2018 edition
diff --git a/src/attr.rs b/src/attr.rs
index da12047..d1e7dca 100644
--- a/src/attr.rs
+++ b/src/attr.rs
@@ -69,7 +69,7 @@
/// [`Attribute::parse_outer`]: #method.parse_outer
/// [`Attribute::parse_inner`]: #method.parse_inner
///
- /// ```
+ /// ```edition2018
/// use syn::{Attribute, Ident, Result, Token};
/// use syn::parse::{Parse, ParseStream};
///
@@ -470,7 +470,7 @@
/// For example if we are developing an attribute macro that is intended to be
/// invoked on function items as follows:
///
-/// ```rust
+/// ```edition2018
/// # const IGNORE: &str = stringify! {
/// #[my_attribute(path = "/v1/refresh")]
/// # };
@@ -482,7 +482,7 @@
/// The implementation of this macro would want to parse its attribute arguments
/// as type `AttributeArgs`.
///
-/// ```rust
+/// ```edition2018
/// extern crate proc_macro;
///
/// use proc_macro::TokenStream;