Fix compiling the doc test without the chrono feature enabled

diesel/src/expression/mod.rs

@@ -590,6 +590,7 @@ where
 ///     name: String,
 ///     #[diesel(select_expression = posts::columns::title)]
 ///     title: String,
+/// #   #[cfg(feature = "chrono")]
 ///     #[diesel(select_expression = diesel::dsl::now)]
 ///     access_time: chrono::DateTime<chrono::Utc>,
 /// }
@@ -606,6 +607,7 @@ where
 ///     id: 1,
 ///     name: "Sean".into(),
 ///     title: "My first post".into(),
+/// #   #[cfg(feature = "chrono")]
 ///     access_time: first_user_post.access_time,
 /// };
 /// assert_eq!(expected_user_post, first_user_post);