Command `ptbench datamodule list` does not completely list the resource name

Due to an upcoming change in #27 (closed), data modules will now be declared with Python module variables instead of multiple files (that will substantially reduce the number of files and documents to maintain). As of now, the names of the Python module attributes related to a datamodule are not showing when the user lists datamodules available:

$ ptbench datamodule list
...
- shenzhen: "ptbench.data.shenzhen.datamodules"  # <- there should be a ":default" after this.
...