tutorial: Surround the snap version in quotes

docs/tutorial/snapcraft.md

@@ -110,7 +110,7 @@ Let's look at an example:
 
 ```yaml
 name: myApp
-version: 2.0.0
+version: '2.0.0'
 summary: A little description for the app.
 description: |
  You know what? This app is amazing! It does all the things