logging.html.in: fix number of output formats available

There are 4 formats available (x:stderr, x:syslog:name,
x:file:file_path, x:journald), not 3. Use "the following"
instead of the actual number to avoid the need to update
the number every time a new form is added/removed.
Suggested-by: NPino Toscano <ptoscano@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

docs/logging.html.in

@@ -124,7 +124,7 @@ x:name</pre>
        priority level, messages that match that filter will still be logged,
        while others will not. In order to see those messages, you must also have
        an output defined that includes the priority level of your filter.</p>
-    <p>The format for an output can be one of those 3 forms:</p>
+    <p>The format for an output can be one of the following forms:</p>
     <ul>
       <li><code>x:stderr</code> output goes to stderr</li>
       <li><code>x:syslog:name</code> use syslog for the output and use the