Correct HACKING.md to refer to work-on
Correct HACKING.md to refer to work-on

reflex-frp/reflex-platform,paldepind/try-reflex,reflex-frp/reflex-platform,ryantrinkle/try-reflex,reflex-frp/reflex-platform,ocharles/try-reflex,ajnsit/reflex-platform