Fix KeyError when running document command

[ljvmiranda921]

Fixes a missing key when running the document command.
Related to #73

Description

I tried running spacy project document -o README.md on an old project (pre-weasel) but arrived at an error:

Traceback (most recent call last):
  File "/home/lj/.local/bin/spacy", line 8, in <module>
    sys.exit(setup_cli())
  File "/home/lj/.local/lib/python3.8/site-packages/spacy/cli/_util.py", line 87, in setup_cli
    command(prog_name=COMMAND)
  File "/home/lj/.local/lib/python3.8/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/home/lj/.local/lib/python3.8/site-packages/typer/core.py", line 778, in main
    return _main(
  File "/home/lj/.local/lib/python3.8/site-packages/typer/core.py", line 216, in _main
    rv = self.invoke(ctx)
  File "/home/lj/.local/lib/python3.8/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/lj/.local/lib/python3.8/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/lj/.local/lib/python3.8/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/lj/.local/lib/python3.8/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/lj/.local/lib/python3.8/site-packages/typer/main.py", line 683, in wrapper
    return callback(**use_params)  # type: ignore
  File "/home/lj/.local/lib/python3.8/site-packages/weasel/cli/document.py", line 47, in project_document_cli
    project_document(project_dir, output_file, no_emoji=no_emoji)
  File "/home/lj/.local/lib/python3.8/site-packages/weasel/cli/document.py", line 115, in project_document
    "start": MARKER_START.format(marker_tag),
KeyError: 'tag'

Types of change

Bugfix

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the test suite, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[adrianeboyd]

Thanks, Lj!

I can confirm that this fixes the initial bug. I see that it still has some problems similar to #72 where it clobbers all the "spaCy project" mentions with "Weasel" in the README, too, which probably isn't what users would expect if they're running spacy project.