rebase: fix incorrect configuration example

This configuration example doesn't make rebase require a destination,
even though help document wants to show such example.

hgext/rebase.py

@@ -668,7 +668,7 b' def rebase(ui, repo, **opts):'
     option:
 
       [commands]
-      rebase.requiredest = False
+      rebase.requiredest = True
 
     Return Values: