Minor bug building list of already displayed words

Because
https://github.com/johniak/word-clock/blob/ea15b364374213c0eb13bf50f3ac40784b4ba7ba/raspberry-pi/src/wordclock/word_clock.py#L71

sets the variable `all_highlighted_words` then

https://github.com/johniak/word-clock/blob/ea15b364374213c0eb13bf50f3ac40784b4ba7ba/raspberry-pi/src/wordclock/word_clock.py#L75

should probably be **+=** and not just **=**.