Update integration.md

[patacra]

Correct the CKEditor integration documentation

Summary of the change:

• In "Sample 2 - With JQuery Selector", the JQuery selector is wrong as the dot is for selecting a class,
  and this class is missing and set on the id instead.
• To help people understand, I propose to modify the id to show that it has to be unique if you use this way of
  setting up the editor. Personally, I prefer the class version, which will handle multiple editors in the page.