doc: graph.h: do not link to `man7.org` for `babeltrace(1)`

doc: graph.h: do not link to `man7.org` for `babeltrace(1)`

This website uses the project's upstream repository to find its manual
pages:

> This page was obtained from the project's upstream Git repository
> ⟨git://git.efficios.com/babeltrace.git⟩ on 2019-05-09.

At that date, the Babeltrace 2 CLI's name was still `babeltrace`, so
<http://man7.org/linux/man-pages/man1/babeltrace.1.html> is actually an
old Babeltrace 2 CLI manual page.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I2a449dd3afee25dfb87993ee0d653f717b109c99
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2871