Please find a solution, depending on whether you want to remove or update the names of the groups

##### Update the names (please note that it also update the top legend)
ggsurvplot(result, risk.table=T, legend.labs=c("a", "b", "c", "d"))

##### Remove the names (please note that it doesn't affect the legend)
ggsurvplot(result, risk.table=T, tables.y.text=FALSE)