Fix translation on frontpage

[Sander Maijers]

I can't trace back the text on https://flexboxfroggy.com/#nl to this repo. It contains a grammatical error which I'd like to fix.

[Thomas Park]

Hey @sanmai-NL, it's broken up into these 3 files:

• levels.js - instructions text
• docs.js - tooltips text
• messages.js - UI text