fix: correct array methods article.md and task.md ru

[front42]

Array methods

• There is no point in trying to change the meaning of the word "между" with any formulas. Let it be more precise — and also regarding commas here & there.
• Negative indexes in methods are interpreted as arr.length + index, so arr.length - 1 is not the position before the last element (where the second-to-last element is) — but the position of the last element.
• A more appropriate term to describe an object’s fields, without ambiguity and in line with common terminology, is properties.