Update integration.md

[patacra]

Correct the CKEditor integration documentation

Summary of the change:

• The JQuery selector was probably wrong as the dot is for selecting a class, and this class was 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.