Fix typo and clarify

ease/private/invert.scrbl

@@ -6,6 +6,9 @@
 @defproc[(ease-invert [easing ease/c]) ease/c]{
   Returns an easing that is like @racket[easing], but
   with proportional changes inverted from @racket[easing].
+  If @racket[easing] starts out proportionally further
+  along than @racket[linear-ease], then the returned
+  easing will start out proportionally behind @racket[linear-ease].
   @ease-examples[
     (define (quadratic-ease x) (* x x))
     (ease-real quadratic-ease 0 100 5)
@@ -14,7 +17,7 @@
 
 @defproc[(ease-reverse [easing ease/c]) ease/c]{
   Returns an easing that is like @racket[easing], but
-  with it's steps reversed.
+  with its steps reversed.
   @ease-examples[
     (ease-real linear-ease 0 100 5)
     (ease-real (ease-reverse linear-ease) 0 100 5)