[whatwg/dom] Clarification on valid shadow element's local name (#869)

As discussed in #745, I made the required changes for clarification on the shadow element's local name.
It follows the formate specified in https://github.com/whatwg/dom/issues/745#issuecomment-639329890 by @annevk.

A side note:
There was an error during the `make` process, on both unedited and edited versions of dom.bs

```txt
Error running preprocessor, returned code: 1.
LINK ERROR: No 'dfn' refs found for 'case-sensitive' that are marked for export.
<a data-link-type="dfn" data-lt="case-sensitive">case-sensitive</a>
 ✘  Did not generate, due to fatal errors
make: *** [Makefile:5: remote] Error 22
```

Due to which I couldn't verify my changes, so please let me know if something is off : )


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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/869


-- Commit Summary --

  * clarification of valid shadow elemet local name

-- File Changes --

    M dom.bs (49)

-- Patch Links --

https://github.com/whatwg/dom/pull/869.patch

https://github.com/whatwg/dom/pull/869.diff


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