replace deprecated String.prototype.substr()
replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated
Signed-off-by: Tobias Speicher <0bf48b1b307ef620053f01d201166f9ed9f81d78@gmail.com>

10xEngineer/reveal.js,hakimel/reveal.js,lvh/reveal.js,lvh/reveal.js,hakimel/reveal.js,10xEngineer/reveal.js