Set all documentation examples to 2018 edition
diff --git a/src/ext.rs b/src/ext.rs
index 38c7a2c..ca6d70d 100644
--- a/src/ext.rs
+++ b/src/ext.rs
@@ -17,7 +17,7 @@
     /// This is useful when parsing a DSL which allows Rust keywords as
     /// identifiers.
     ///
-    /// ```rust
+    /// ```edition2018
     /// use syn::{Error, Ident, Result, Token};
     /// use syn::ext::IdentExt;
     /// use syn::parse::ParseStream;