* lib/net/ftp.rb: [DOC] Document Net::FTP.mdtm and .set_socket and fix
  spelling typo, based on patch by @artfuldodger [Fixes GH-426]
  https://github.com/ruby/ruby/pull/426


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43571 b2dd03c8-39d4-4d8f-98ff-823fe69b080e