Set all documentation examples to 2018 edition
diff --git a/src/group.rs b/src/group.rs
index d2cbd3e..5982a8f 100644
--- a/src/group.rs
+++ b/src/group.rs
@@ -94,7 +94,7 @@
///
/// # Example
///
-/// ```rust
+/// ```edition2018
/// # extern crate quote;
/// # use quote::quote;
/// #
@@ -152,7 +152,7 @@
///
/// # Example
///
-/// ```rust
+/// ```edition2018
/// # extern crate quote;
/// # use quote::quote;
/// #
@@ -231,7 +231,7 @@
///
/// # Example
///
-/// ```rust
+/// ```edition2018
/// # extern crate proc_macro2;
/// # extern crate quote;
/// #