message: update outdated message in ssr plugin
diff: -const { red, yellow, gray } = require('chalk')
  +const { red, yellow } = require('chalk')
  const prefix = `[vue-server-renderer-webpack-plugin]`
  const warn = exports.warn = msg => console.error(red(`${prefix} ${msg}\n`))
  @@ -15,9 +15,8 @@ export const validate = compiler => {
  if (!compiler.options.externals) {
  tip(
  - 'It is recommended to externalize dependencies for better ssr performance.\n' +
  - `See ${gray('https://github.com/vuejs/vue/tree/dev/packages/vue-server-renderer#externals')} ` +
  - 'for more details.'
  + 'It is recommended to externalize dependencies in the server build for ' +
  + 'better build performance.'
  )
  }
  }
  