Fixed various spelling errors.

original commit: c9519fd11300b07292df8619e86f48533eff6e4d

collects/mzlib/kw.rkt

@@ -128,7 +128,7 @@
        [else (ormap (lambda (k) (and (assq k rests) #t)) ; suggested?
                     (car (cddddr processed-spec)))])))
   (define (make-keyword-get-expr key rest default known-vars)
-    ;; expand (for id macros) and check if its a simple expression, because if
+    ;; expand (for id macros) and check if it's a simple expression, because if
     ;; it is, evaluation cannot have side-effects and we can use keyword-get*
     (define default*
       (local-expand default 'expression (cons #'#%app known-vars)))

collects/mzlib/thread.rkt

@@ -7,7 +7,7 @@
 
   #|
   t accepts a function, f, and creates a thread. It returns the thread and a
-  function, g. When g is applied it passes it's argument to f, and evaluates
+  function, g. When g is applied it passes its argument to f, and evaluates
   the call of f in the time of the thread that was created. Calls to g do not
   block.
   |#