erb.rb: ERB#result does not accept a proc.

* lib/erb.rb (result): [DOC] no longer accepts a Proc, as
  Kernel.eval does not.  [fix GH-619]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46241 b2dd03c8-39d4-4d8f-98ff-823fe69b080e