Remove outdated example from Numeric documentation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/branches/ruby_2_5@64435 b2dd03c8-39d4-4d8f-98ff-823fe69b080e