docs: change data with state (#2980)

Every piece in the object that returns `state` is what must be declared.
Even though we can think of `state` *as* the `data` of the store, but I believe that is confussing to use both terms indistinctly.

Author: leoelz

packages/docs/core-concepts/state.md

@@ -32,7 +32,7 @@ export const useStore = defineStore('storeId', {
 
 :::tip
 
-In order for Vue to properly detect state, you must declare every state piece in `data`, even if its initial value is `undefined`.
+In order for Vue to properly detect state, you must declare every state piece in `state`, even if its initial value is `undefined`.
 
 :::