remove unnecessary string interpolation.
remove unnecessary string interpolation.

SteamShon/incubator-s2graph,daewon/incubator-s2graph,daewon/incubator-s2graph,SteamShon/incubator-s2graph,jongwook/incubator-s2graph,jongwook/incubator-s2graph,SteamShon/incubator-s2graph