docs: class properties and object spread are no longer experimental (#4754)

Co-authored-by: Romain Bohdanowicz <romain.bohdanowicz@formation.tech>

content/tutorial/tutorial.md

@@ -542,7 +542,7 @@ var player = {score: 1, name: 'Jeff'};
 var newPlayer = Object.assign({}, player, {score: 2});
 // Now player is unchanged, but newPlayer is {score: 2, name: 'Jeff'}
 
-// Or if you are using object spread syntax proposal, you can write:
+// Or if you are using object spread syntax, you can write:
 // var newPlayer = {...player, score: 2};
 ```