Fixed a typo in urlencode documentation.

docs/ref/utils.txt

@@ -698,7 +698,7 @@ escaping HTML.
 .. function:: urlencode(query, doseq=0)
 
     A version of Python's urllib.urlencode() function that can operate on
-    unicode strings. The parameters are first case to UTF-8 encoded strings
+    unicode strings. The parameters are first cast to UTF-8 encoded strings
     and then encoded as per normal.
 
 .. function:: cookie_date(epoch_seconds=None)