docs: Fix gir generating warning
It fixes documentation on ges-marker-list signals.
../subprojects/gst-editing-services/ges/ges-marker-list.c:165: Warning: GES: incorrect number of parameters in comment block, parameter annotations will be ignored. ../subprojects/gst-editing-services/ges/ges-marker-list.c:191: Warning: GES: incorrect number of parameters in comment block, parameter annotations will be ignored.