Document training subcommands

Documentation about training subcommands were missing.

dan/ocr/line/__init__.py

@@ -16,7 +16,7 @@ def add_generate_parser(subcommands) -> None:
 def add_line_parser(subcommands) -> None:
     parser = subcommands.add_parser(
         "line",
-        description="Train a DAN model at line level",
+        description="Train a DAN model at line level.",
         help="Train a DAN model at line level.",
     )
     parser.set_defaults(func=run_train)