Remove outdated example from Numeric documentation

Since 2.5, Numeric instances can be cloned and duplicated.

[Fix GH-1850]

From: Miguel Landaeta <miguel@miguel.cc>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63112 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

numeric.c

@@ -5277,10 +5277,11 @@ rb_int_s_isqrt(VALUE self, VALUE num)
  *   1.object_id == a.object_id   #=> true
  *
  * There can only ever be one instance of the integer +1+, for example. Ruby ensures this
- * by preventing instantiation and duplication.
+ * by preventing instantiation. If duplication is attempted, the same instance is returned.
  *
- *   Integer.new(1)   #=> NoMethodError: undefined method `new' for Integer:Class
- *   1.dup            #=> TypeError: can't dup Integer
+ *   Integer.new(1)                   #=> NoMethodError: undefined method `new' for Integer:Class
+ *   1.dup                            #=> 1
+ *   1.object_id == 1.dup.object_id   #=> true
  *
  * For this reason, Numeric should be used when defining other numeric classes.
  *