Fix error in readme intro (remove "know")

The ever-astute @trotzig pointed out[1] that this word does not belong
here. I missed it when editing the intro he had cooked up.

[1]: https://github.com/trotzig/diffux/commit/eaa844710746b00dfc09f1ee9d4b274e6bb3e9d4#commitcomment-5291194

Change-Id: Id2f0653cf2198be3fc40cba1de6376bf7e5e6805