Fix slightly misleading instructions.

I don't think `require opal-jquery` needs to be `require`d explicitly for the DOM manipulation example. However, `Document.ready?` should be in there.
Re-add require 'opal-jquery'
Added missing 'do'
Merge pull request #1 from benjamintanweihao/patch-2

Added missing 'do'
Merge pull request #22 from benjamintanweihao/patch-1

Fix slightly misleading instructions.