doc: fix 'git status --help' character quoting

Correct backtick quoting for some of the modification states to give
consistent web rendering.

While at it, use 00 for ASCII NUL to avoid any confusion with the letter O.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
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Noticed when reviewing a stackoverflow problem where the OP actually
desired to just list those files with merge indicators present.