chore: update index.js (#938)

Updated URL of docs (v0.vuepress.vuejs.org) to fix broken link

Author: thiemo

lib/prepare/index.js

@@ -36,7 +36,7 @@ module.exports = async function prepare (sourceDir) {
   if (hasUserOverride && !hasUserStyle) {
     logger.tip(
       `${chalk.magenta('override.styl')} has been split into 2 APIs, we recommend you upgrade to continue.\n` +
-      `      See: ${chalk.magenta('https://vuepress.vuejs.org/default-theme-config/#simple-css-override')}`
+      `      See: ${chalk.magenta('https://v0.vuepress.vuejs.org/default-theme-config/#simple-css-override')}`
     )
   }