add a missing "not" in bug2331 changelog

changes/bug2331

@@ -1,6 +1,7 @@
   o Minor bugfixes:
     - Add assertions to check for overflow in arguments to
       base32_encode and base32_decode; fix a signed-unsigned
-      comparison there too.  These bugs are actually reachable in Tor,
-      but it's good to prevent future errors too.  Found by doorss.
+      comparison there too.  These bugs are not actually reachable in
+      Tor, but it's good to prevent future errors too.  Found by
+      doorss.