Corrected typo
Corrected typo

Corrected typo on line 134 and slightly improved explanations about `refreshIconOptions()` method
nerik/Leaflet.markercluster,zverev/Leaflet.markercluster,frenomus/Leaflet.markercluster,604557209/byzgf,ghybs/Leaflet.markercluster,nerik/Leaflet.markercluster,Leaflet/Leaflet.markercluster,zverev/Leaflet.markercluster,604557209/byzgf,ghybs/Leaflet.markercluster,Leaflet/Leaflet.markercluster,frenomus/Leaflet.markercluster