Update 03_observer.md

recieve -> receive[x2] -If this is a us/gb thing please let me know
forwarned -> forewarned
benifit -> benefit
caefully -> carefully
allowng -> allowing
peice -> piece
inadvertantly -> inadvertently
problemattic -> problematic[x2]
aformentioned -> aforementioned
approriately -> appropriately
recieving -> receiving[x2] - us/gb ?
receieve - > receive
inadvertandly -> inadvertently