wording correction

[EbubeTheSage]

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Sub-chapter on Variable declaration contains:

"In Nim we also distinguish values that can change, or mutate, from those that can’t"

The sub-chapter is focused on using var to signify a variable's mutability, not a value's (an object).

Teaching new coders to equate variables (references) to objects will cause a lot of issues downstream.

Fixing it would require a simple change of the word 'values' to 'variables', as a hyperlink to an easy-to-understand article on variables vs. objects.

Furthermore, later on, on object mutability, we can introduce distinctions between mutable & immutable types.