Fix typo in readme

These are needed to avoid the following error:

```
Invalid CSS after "...trap-sprockets"": expected selector or at-rule, was "@import "bootst..."
  (in /Users/zacharyadams/Dropbox/launchacademy/food_trucks/app/assets/stylesheets/application.css.scss:2)
```
Merge pull request #652 from ZacharyAdams/master

Fix typo in readme