Doomsday Algorithm: Fix leap year check

[minh-swinburne]

Replace != in (year % 400) != 0 (line 49) with ==

Justification: Years that are divisible by 100 (centurian == 100) but not by 400 (year % 400 != 0) are skipped and NOT leap year.

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[imSanko]

Parenthesize a and b expressions when chaining and and or together, to make the precedence clear

[minh-swinburne]

I've changed the incorrect conditional operator and ensure clarity using parentheses

[tianyizheng02]

Suggested change

	if (year % 4 != 0) or ((centurian == 0) and (year % 400 != 0))
	if year % 4 != 0 or (centurian == 0 and year % 400 != 0)

I believe some of the parentheses are unnecessary, order of operations should still be clear due to the higher precedence of %, ==, and !=.