docs: fix link to an example file
docs: fix link to an example file

Relative links within the source tree work, but they were not being
turned into nice usable hyperlink that go back to GitHub under the
generated documentation.

Reported-by: Melin, Stefan M. <f6812e94e093f79362ff400f82f84b6ee153d97d@teliacompany.com>
Change-Id: I137ad95fa75a6ee5e1b03a252782e6357d36a3af

Telecominfraproject/gnpy