update_menu clarification

[ykharitonova]

the inconsistency between the project instructions and the docstring for the update_menu_dish function. the docstring for the update_menu_dish function states that
If the field_key is invalid, return -2.
however, it seems that the autograder is expecting the "field_key" as the correct return value in this case.

Explanation:
In the docstring, when we said "If the field_key is invalid, return -2", we meant that if the name of the field itself is invalid, then return -2.
If the field_key is valid, but the corresponding field_info is invalid, then we return the name of the field.