Merge pull request #3955 from joncfoo/patch-1

Document $: in the tutorial

site/content/tutorial/02-reactivity/02-reactive-declarations/text.md

@@ -11,7 +11,7 @@ let count = 0;
 $: doubled = count * 2;
 ```
 
-> Don't worry if this looks a little alien. It's valid (if unconventional) JavaScript, which Svelte interprets to mean 're-run this code whenever any of the referenced values change'. Once you get used to it, there's no going back.
+> Don't worry if this looks a little alien. It's [valid](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label) (if unconventional) JavaScript, which Svelte interprets to mean 're-run this code whenever any of the referenced values change'. Once you get used to it, there's no going back.
 
 Let's use `doubled` in our markup: