Fix regression in PCD8544.js

flip() was broken by https://github.com/espruino/EspruinoDocs/commit/0c7cd947c08e93020faae3e233d7d1bf97bb9fe3#diff-54e8aad38d317a8ea7e056d928e39c43 

It seems that you do need those two bytes at the end of each line - except on the last line, where they'll overwrite the first 2 bytes of the first line. Weird! Seems to work like this.