Set all documentation examples to 2018 edition
diff --git a/src/error.rs b/src/error.rs
index e3bc304..6a1a200 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -51,7 +51,7 @@
     ///
     /// # Example
     ///
-    /// ```
+    /// ```edition2018
     /// use syn::{Error, Ident, LitStr, Result, Token};
     /// use syn::parse::ParseStream;
     ///