chore: fix some legacy doc urls in warnings and readme (#12725) [ci skip]

Author: Justineo

README.md

@@ -73,7 +73,7 @@ Vue.js supports all browsers that are [ES5-compliant](https://kangax.github.io/c
 
 ## Documentation
 
-To check out [live examples](https://vuejs.org/v2/examples/) and docs, visit [vuejs.org](https://vuejs.org).
+To check out [live examples](https://v2.vuejs.org/v2/examples/) and docs, visit [vuejs.org](https://v2.vuejs.org).
 
 ## Questions
 

src/core/instance/proxy.ts

@@ -19,7 +19,7 @@ if (__DEV__) {
         'referenced during render. Make sure that this property is reactive, ' +
         'either in the data option, or for class-based components, by ' +
         'initializing the property. ' +
-        'See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.',
+        'See: https://v2.vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.',
       target
     )
   }
@@ -29,7 +29,7 @@ if (__DEV__) {
       `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. ' +
-        'See: https://vuejs.org/v2/api/#data',
+        'See: https://v2.vuejs.org/v2/api/#data',
       target
     )
   }

src/core/instance/state.ts

@@ -127,7 +127,7 @@ function initData(vm: Component) {
     __DEV__ &&
       warn(
         'data functions should return an object:\n' +
-          'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
+          'https://v2.vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
         vm
       )
   }