Use relative paths in train.txt and val.txt

[thewh1teagle]

When doing pre processing val.txt and train.txt created. but they have absolute paths configured.
Please use relative paths to the json and wav files so I can share the pre-processed dataset with other people.

[mush42]

@thewh1teagle I write absolute paths to be able to simlink the two files to the data directory in the repo while storing the data itself elsewhere. Feel free to make a PR for this.