doc: Corrected example in 17.8 Deriving implementations for traits

Corrected example to to use Rand trait referenced in preceding
description and included an example using the Show trait to print ABC.

src/doc/tutorial.md

@@ -2584,11 +2584,18 @@ for `Eq` and can be used with the equality operators, and that a value
 of type `ABC` can be randomly generated and converted to a string:
 
 ~~~
+extern crate rand;
+
 #[deriving(Eq)]
 struct Circle { radius: f64 }
 
-#[deriving(Clone, Show)]
+#[deriving(Rand, Show)]
 enum ABC { A, B, C }
+
+fn main() {
+    // Use the Show trait to print "A, B, C."
+    println!("{}, {}, {}", A, B, C);
+}
 ~~~
 
 The full list of derivable traits is `Eq`, `TotalEq`, `Ord`,