Fix the bug in `hands_on/bug_hunt/file_datastore.py`

[pberkes]

The FileDatastore is initialized using a base_path, and then it should be able to read/write files in it, given a path relative to the base path.

The current code works correctly when base_path is an absolute path but not when it's a relative path!

Go to hands_on/bug_hunt and run file_datastore.py to reproduce the bug. Use the debugger to find the bug the fix it.