Updated broken link in the README.md

Sure, here's a more concise and clear version:

"The old variable link (https://bulma.io/documentation/overview/variables/) was broken. I used the Wayback Machine to see where it originally directed and found that the variables page was divided into different sections. The new link is now (https://bulma.io/documentation/customize/#docsNav), which I have updated in the README."

Is this better?