Make text node type more explicit

Instead of placing magic numbers around, I feel this should better portray what this is.

site/content/tutorial/10-transitions/05-custom-js-transitions/text.md

@@ -8,7 +8,7 @@ While you should generally use CSS for transitions as much as possible, there ar
 function typewriter(node, { speed = 50 }) {
 	const valid = (
 		node.childNodes.length === 1 &&
-		node.childNodes[0].nodeType === 3
+		node.childNodes[0].nodeType === Node.TEXT_NODE
 	);
 
 	if (!valid) {
@@ -26,4 +26,4 @@ function typewriter(node, { speed = 50 }) {
 		}
 	};
 }
-```
+```