Doc: Removed an unnecessary page command from the class documentation

QDoc failed to generate docs for QMacToolBar as its class
documentation had an unnecessary \fn instance within the \class
comment block. Replaced the \fn instance with \l.

Change-Id: Id36e6c9749f3d0a08c0d2a6dd73cb9bd44cac0af
Task-number: QTBUG-40459
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>