[recipes] Correct CSS pre-processors section

Created by: donovanmuller

As someone new to Gulp and still finding my feet, I naively copy and pasted the code from the CSS pre-processors section thinking it would just work (following the same conventions for directory layout as described in the simple example).

It does not just work however

• the scss directory is relative to the base directory (which is not the convention set out in the previous example)
• the sass tasks paths also similarly do not follow convention
• running just grunt serve as per the simple example does not invoke the sass task by default. (Of course you can run grunt sass serve but it seems more convenient if I could just run the same grunt serve as in the simple example - not sure if this is bad practice or not?)

Given the above, I've corrected the paths and add the sass task as a dependency to serve. I've also filled in the blanks just to make things as clear as possible for someone getting into Gulp. I.e. just copy, paste and run.