Docs: Correct lttng-ust man page references

Docs: Correct lttng-ust man page references

==== Observed issue ====

Deploying the lttng-www site fails with the following broken link:

```
urlname;parentname;base;result;warningstring;infostring;valid;url;line;column;name;dltime;size;checktime;cached;level;modified
/man/1/lttng-ust/v2.13;http://localhost:10000/man/8/lttng-sessiond/v2.13/;;404
Not
Found;;;False;http://localhost:10000/man/1/lttng-ust/v2.13;730;8;lttng-ust(1);-1;160;3.2490103244781494;0;3;
```

==== Cause ====

LTTng-UST does not ship a man page in section 1[1].

==== Solution ====

Correct the lttng-ust man page references in `lttng-sessiond.8.txt`.

==== Know drawbacks ====

None.

==== References ====

[1]: https://github.com/lttng/lttng-ust/tree/5cc8729236a95d784e9561abbcb93a0fce90890c/doc/man

Change-Id: If697be273a9c4db38a7723adc469f45818bad355
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>