[css-color-4] fix typo in conversions.js

[romainmenke]

[css-color-4] fix typo

Current code behaves like this (which I think is a typo) :

var alpha = 1;
if (alpha = 0) {
 console.log('truthy')
} else {
 console.log(5 / alpha);
}

> Infinity

[w3cbot]

svgeesus marked as non substantive for IPR from ash-nazg.

[svgeesus]

Typo indeed, thanks!