Add named argument in bst2html.py to fix documentation

Add named argument in bst2html.py to fix documentation

On my system, make -C doc will give this error:

File "/home/jimmacarthur/bs/buildstream/doc/bst2html.py", line 443, in <module>
  @click.argument('description', click.Path(file_okay=True, dir_okay=False, readable=True))
...
TypeError: Arguments take exactly one parameter declaration, got 2

Proposed change

Changes proposed in this merge request:

• Name the second argument to @click.argument 'type'

No issue raised because I believe this fix is trivial.

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