Update motion.md
Update motion.md

Replace UTF-8 incompatible <<â€™>> (prime symbol) with <<'>> (typewriter apostrophe). I suspect Windows-1252 character encoding is at fault for this. Replace both <<â€œ>> (opening quotation mark) and <<â€>> (closing quotation mark) with <<">> (quotation mark). Rephrase things to make sense in a question and answer format. Introduce and explain what Box2D is, alleviating confusion and providing a reason for bringing it up in the first place. Capitalize word to ensure consistency. Speed is a scalar quantity, whereas velocity is a [sic] Euclidean vector.
Stencyl/stencylpedia