It's not ideal to modify slots directly, so the more proper alternative would be to convert it into a data.frame with as.data.frame(kk2x) and manually make the plot with ggplot2. Just know that slot names are not guaranteed to be documented in package updates, so may break in the future.
The first example is looking for the hyphen and splitting the character vector then selecting the first chunk which corresponds to your pathway. The second is directly replacing the hyphen and species with an empty character. Functionally equivalent, but the latter requires manipulation if you were to change species.