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

include/babeltrace2/graph/graph.h

@@ -429,7 +429,7 @@ graph topologies. They are:
         to a file.
 
         This is equivalent to the \c text output format of the
-        <a href="http://man7.org/linux/man-pages/man1/babeltrace.1.html">Babeltrace&nbsp;1
+        <a href="https://manpages.debian.org/stretch/babeltrace/babeltrace.1.en.html">Babeltrace&nbsp;1
         command-line tool</a>.
 
         See \bt_man{babeltrace2-sink.text.pretty,7}.