Fix documentation warnings.

doc/src/platforms/windows.qdoc:234: warning: Unknown command '\cQt'
doc/src/platforms/windows.qdoc:235: warning: Unknown command '\cQt'
doc/src/platforms/windows.qdoc:236: warning: Unknown command '\cQt'
doc/src/platforms/windows.qdoc:261: warning: Unknown command '\cQt'

Change-Id: Ideae735d9000384e41b909efa7ba96cc5891f770
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>