doc: fix viewer example code listing.
doc: fix viewer example code listing.

Squareys/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,Squareys/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,mosra/magnum-examples