Fix typos in Ch04

existance -> existence
usecase -> use case
assoication -> association
legnth -> length
Therefor -> Therefore
turn -> turns
identifers -> identifiers
trouble shooting -> troubleshooting
did we do -> we did
realy -> really
numberi -> cnumeric
distigiush -> distinguish
seens -> seen
self optimizingself-optimizing
chatper -> chapter
uqnieu -> unique
algorith -> algorithm
presreve -> preserve
non consecutivenonconsecutive
sevearl -> several
responce -> response
explcitly -> explicitly
conceuptial -> conceptual
prdocuts -> products
seprate -> separate
assoicated -> associated
efficent -> efficient
removed a "is is"
added some -s