Update example to not use ^ as a hash function

XOR is not a good hash function, and our example shouldn't give people the wrong idea. Better to just outsource to the built-in Hash function used by `Array`. It's more idiomatic, and will lead to less collisions.

README.md

@@ -120,7 +120,7 @@ class Email
   end
 
   def hash
-    self.class.hash ^ address.hash
+    [self.class, address].hash
   end
 end
 
@@ -144,7 +144,7 @@ class Phone
   end
 
   def hash
-    self.class.hash ^ country.hash ^ number.hash
+    [self.class, country, number].hash
   end
 end
 ```