Fix typo in UPGRADING for 7.2

Author: issei-m

UPGRADING

@@ -25,7 +25,7 @@ PHP 7.2 UPGRADE NOTES
   . is_object() will now return true for objects of class
     __PHP_Incomplete_Class.
   . Support for Netware operating systems have been removed.
-  . Casting arrays to objects (with (object) or settype()) will now covert
+  . Casting arrays to objects (with (object) or settype()) will now convert
     integer keys to string property names. This fixes the behaviour of previous
     versions, where integer keys would become inaccessible properties with
     integer names.