set watch option as flush: 'post'

in some cases useTitle doesn't work .
it has to  work in nextick 

https://codepen.io/yjl626/pen/rNyKVZW
vuejs/core#2730

set watch option as flush: 'post' can fixed it
I think it's a reliable option in this function

Author: YJL626

packages/core/useTitle/index.ts

@@ -35,7 +35,10 @@ export function useTitle(
       if (isString(t) && t !== o && document)
         document.title = t
     },
-    { immediate: true },
+    { 
+      immediate: true ,
+      flush: 'post'
+    },
   )
 
   if (observe && document) {