Update scala-features.md

[stensh1]

The sentence "Например, переменные создаются лаконично, а их типы понятны:" does not correspond to the code example, there are no "var" but "val" instead.

[som-snytt]

vals and vars are both called variables in an informal context. Otherwise, "mutable variable" would be redundant. Other distinctions I would prefer be universally honored include "definition" vs "declaration", and "its" vs "it's". Also a constant is a constant only if it is a mathematically precise value that never changes.