chore: fix prefixed data properties warning typo (#10327)

src/core/instance/proxy.js

@@ -28,7 +28,7 @@ if (process.env.NODE_ENV !== 'production') {
     warn(
       `Property "${key}" must be accessed with "$data.${key}" because ` +
       'properties starting with "$" or "_" are not proxied in the Vue instance to ' +
-      'prevent conflicts with Vue internals' +
+      'prevent conflicts with Vue internals. ' +
       'See: https://vuejs.org/v2/api/#data',
       target
     )