Update pre-processors.md

[gemmadlou]

I discovered the <style lang="sass"> does not work and the following error is created. It may be something isolated to my build.

However:

!./src/components/MyNav.vue
Module build failed: 
    .my-nav {
        ^
      Invalid CSS after ".my-nav {": expected "}", was "{"
      in /Users/gemmablack/Sites/hello/src/components/MyNav.vue (line 29, column 10)
 @ ./~/vue-style-loader!./~/css-loader!./~/vue-loader/lib/style-rewriter.js?id=data-v-3a9c6774&scoped=true!./~/sass-loader?indentedSyntax!./~/vue-loader/lib/selector.js?type=styles&index=0!./src/components/MyNav.vue 4:14-289 13:2-17:4 14:20-295
 @ ./src/components/MyNav.vue
 @ ./~/babel-loader/lib!./~/vue-loader/lib/selector.js?type=script&index=0!./src/App.vue
 @ ./src/App.vue
 @ ./src/main.js
 @ multi main

[kazupon]

Thank you for your PR!

I checked with using wepack-simple template too.
In my enviroment, surely the same error occurred.

I think this issue is in docs.
Already, There is improvment of docs.
#517

I'll try to merge! 😉