fix: use slant side instead of height in parallelogram_perimeter

[https-hari]

Fixes #3174

Problem

parallelogram_perimeter(base, height) used the perpendicular height as
the second argument. The perimeter of a parallelogram is 2*(base + side)
where side is the slant side length — NOT the perpendicular height.

Fix

• Renamed parameter from height to side in function signature
• Updated Doxygen @param doc accordingly
• Updated test (7th) to use int_side variable and correct comment