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

Motivation:
* "domain.com" points to a commercial website
* the domain name "example.com" ("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

Signed-off-by: xchs <github@gmx.net>
Merge pull request #6212 from xchs/change-example-domain