Minor corrections to main and Q&A pages

[mgmalheiros]

In the main page, section Input, the old parameter period is mentioned three times, although the correct now is btnp(key, [hold], [repeat]).

I had to look at the source when upgrading my old 1.5 code.

This change should be also noted in the Q&A page, section How to Migrate to a New Version, along with the removal of the pal parameter from init().

Thank you very much for such nice environment!

[kitao]

Thank you. I replaced "period" in the READMEs.
I'll add explanation for a palette option in Q&A,

[kitao]

I updated the Q&A too.