loadShader(), shader() were missing on listing 4.1

https://github.com/processing/processing-web/issues/144
Add missing loadShader(), shader() instructions

My previous commit fixed the program at 4.1, but introduced a mistake in the flow of the explanation. This commit should deal with that. 

The wording can be probably improved (non native English speaker here).
Merge pull request #385 from hamoid/patch-1

loadShader(), shader() were missing on listing 4.1