Changed unsubscribe from let to const (reduxjs#2582)

Using const is more correct since you don't want unsubscribe to be mutated

Author: omerts

docs/basics/Store.md

@@ -43,7 +43,7 @@ console.log(store.getState())
 
 // Every time the state changes, log it
 // Note that subscribe() returns a function for unregistering the listener
-let unsubscribe = store.subscribe(() =>
+const unsubscribe = store.subscribe(() =>
   console.log(store.getState())
 )