Update mabla.txt

This definition is more useful and matches {mabla}'s usage. Also, Robin has already announced {mabla} and {zabna}'s new definitions official by fiat on the BPFK list[1] once, so might just be a mere formality. 
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[1]: https://groups.google.com/forum/#!topic/bpfk-list/rqMLDfmGBWk