Fix opacity of centroid labels

[moritzschaefer]

There are two bugs that prevent an implement feature (opacity for centroid labels) to work

1. a type here: (opactity instead of opacity)
   

   cellxgene/client/src/components/graph/overlays/centroidLabels.js

   Line 153 in 4281a8f

   opactity={selected ? 1 : deselectOpacity}

2. the use of {} brackets here (the need to be deleted):

   cellxgene/client/src/components/graph/overlays/centroidLabels.js

   Line 208 in 4281a8f

   opacity: { opacity },