Rename joystick inputs

Bring Robotron's joystick naming conventions inline with Smash TV's.

Robotron: 2048 and Smash TV have identical controls as far as I can tell (left joystick to move, right joystick to fire a gun in a particular direction), both being by Eugene Jarvis.  It therefore makes sense if these controls are labelled up consistently.

I've therefore changed src/mame/drivers/williams.c to bring it inline with src/mame/drivers/midyunit.c in this regard.

Please note that I'm not *that* well versed in C, and I'm on a Mac, so I lack the knowledge to compile MAME to test this out.  At any rate, it's more of a suggestion than a fix.