Error in one Diet Planning code line

[JoelPasvolsky]

Description
Diet Planning example line

quantities[0] * quantities[1]

needs to be updated to use non-real quantities

To Reproduce

>>> quantities[0] * quantities[1]
...
ValueError: REAL variables (e.g. 'rice') cannot have interactions

Expected behavior
Use the int variables or note the error

Environment:

• OS: WIN10
• Python version: 3.9
• Ocean SDK version: 6.9.0

Additional context
Reported by user