Docs: Fix template error for a couple links

The [Template engines](http://docs.spring.io/spring-boot/docs/1.1.0.BUILD-SNAPSHOT/reference/htmlsingle/#boot-features-spring-mvc-template-engines) section in the docs had a couple of template errors for links. This fixes those, plus adds a link for Thymeleaf.
![screen](https://cloud.githubusercontent.com/assets/4712580/3009138/85dec5c8-dee9-11e3-88cf-b5d612fc0c15.png)