Change argument name from inner variable name

This edit only affects the Avoid Side Effects (part 2) section.
It replaces the discussion of pass by reference with an explanation in terms of mutability. 
By avoiding the confusing terminology and possible links to the implementation details of 
different JS engines the explanation can be kept at an abstraction level that's great for 
beginners and experts alike. I also cleaned up some grammar, typos, etc.

…-pass-by-reference

Replace pass-by-reference with mutability, clean up

…illiseconds

Add calculation for milliseconds in a day

…-pass-by-reference

Fix stray hyphen turning into a random bullet

Add Serbian translation.

"PER" has the same meaning as "IN_A" but is less verbose

Make example more succinct

Fix wrong naming of `default parameters`

…ation

French translation

Persian translation added (Fluent + RTL styled)