Fix the `num_lit` grammar in the reference manual.

- Cause `0` to be considered a valid integer literal (it is).
- Add octal literals (missed from #10243).

I have *not* modified doc/po/rust.md.pot or doc/po/ja/rust.md.po at all;
they already seem to be out of date so it's easier to ignore them for
myself. I can update them if desired, of course.
auto merge of #10510 : chris-morgan/rust/fix-num_lit-grammar-in-the-reference-manaul, r=cmr

- Cause `0` to be considered a valid integer literal (it is).
- Add octal literals (missed from #10243).

I have *not* modified doc/po/rust.md.pot or doc/po/ja/rust.md.po at all;
they already seem to be out of date so it's easier to ignore them for
myself. I can update them if desired, of course.