Update environment-variables.md

I lost quite a few hours figuring out why my environment variable was `undefined` in the browser.
I read about the built-in support and was like: "Oh nice" and didn't read much further. I missed the part about how you need to prefix your variables with `NEXT_PUBLIC_` in order to expose them to the browser.
I think a hint to this in the anchor link to that section will make it more discoverable as it's then mentioned near the top and will save people who are like me some time and a headache.

docs/basic-features/environment-variables.md

@@ -16,7 +16,7 @@ description: Learn to add and access environment variables in your Next.js appli
 Next.js comes with built-in support for environment variables, which allows you to do the following:
 
 - [Use `.env.local` to load environment variables](#loading-environment-variables)
-- [Expose environment variables to the browser](#exposing-environment-variables-to-the-browser)
+- [Expose environment variables to the browser by prefixing with `NEXT_PUBLIC_`](#exposing-environment-variables-to-the-browser)
 
 ## Loading Environment Variables