docs: Correct GitHub URLs in lttng-ust.3

docs: Correct GitHub URLs in lttng-ust.3

The branches follow the format `stable-X.YZ` rather than `vX.YZ`.

Furthermore, when rendering the man pages from source, the URLs were
omitted completely as the subsitution `{lttng_version}` was not
defined. This hasn't been an issue for the published HTML versions as
those are produced via a different script in the `lttng-www` project
which presumably sets the substitution properly.

Change-Id: Ib96c99df13ddf724e128f95e7ce7c74b2c10c766
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>