Replace "domain.com" by "example.com"

Motivation:
* `domain.com` points to a commercial website
* the domain name `example.com` (as well as `example.net`, `example.org`, and `example.edu`) was reserved for documentation purposes and examples of the use of domain names and is used in a generic and vendor-neutral manner

![install](https://f.cloud.github.com/assets/1218766/1187979/36afc040-23ac-11e3-9939-35789920b196.png)

Curiously, people actually start using the presumed "example" domain name `domain.com` and land on a [website](http://www.domain.com/) that offers commercial services (see e.g. https://community.contao.org/de/showthread.php?44541-Contao-3-1-Unvollst%E4ndige-Installation)

Therefore, I'd suggest to replace the `domain.com` by the reserved domain name `example.com` (http://www.example.com/) which is originally designed for purposes of that kind (https://en.wikipedia.org/wiki/Example.com).