Switch quadratic interaction in diet example to ints

[JoelPasvolsky]

Description
Diet Example has this code:

>>> quantities[0] * quantities[1]          
QuadraticModel({'rice': 0.0, 'tofu': 0.0}, {('tofu', 'rice'): 1.0}, 0.0,
           {'rice': 'REAL', 'tofu': 'REAL'}, dtype='float64')

It should be switched to

>>> quantities[2] * quantities[5]
Out[7]: QuadraticModel({'banana': 0.0, 'avocado': 0.0}, {('avocado', 'banana'): 1.0}, 0.0, {'banana': 'INTEGER', 'avocado': 'INTEGER'}, dtype='float64')   

To Reproduce
See above

Expected behavior
Need int vars, not real

Environment:

• OS: Any
• Python version: Any

Additional context
CC @tmittal7

And remove the # doctest: +SKIP that kept us from catching this earlier.