Update of collision checking example

[jkaniuka]

Hi @petercorke :wave:, this PR fixes issue #376.

The sample code snippet for collision detection listed on your github.io site generates the following errors:

• AttributeError: module 'roboticstoolbox' has no attribute 'Box'
• TypeError: Cuboid.__init__() takes 2 positional arguments but 3 were given
• TypeError: Robot.collided() missing 1 required positional argument: 'shape'
• FutureWarning: method collided is deprecated, use iscollided instead

I decided to fix this code snippet as part of this pull request :sunglasses:

Regards, Jan