You can fetch the main.js via command line following the instructions under “Try out”: https://github.com/graphhopper/graphhopper/blob/master/docs/core/quickstart-from-source.md or you can go to the “JavaScript” section and installing npm & build the main.js from sources.
If you are on master you should probably the more sophisticated PT UI (show here) that currently works without the npm toolchain (only maven). See https://github.com/graphhopper/graphhopper/pull/1760 and e.g. open reader-gtfs/src/main/resources/assets/pt/index.html