Add indexer marker for TWEAK

doc/Language/objects.pod6

@@ -442,6 +442,7 @@ name. In either case, if neither C<BUILD> nor the default mechanism has
 initialized the attribute, default values are applied (the C<2 * $!x> in the
 example above).
 
+X<TWEAK>
 After the C<BUILD> methods have been called, methods named C<TWEAK> are
 called, if they exist, again with all the named arguments that were passed
 to C<new>.