[w3c/gamepad] Editorial: Fixed a few inconsistencies in "GamepadButton Interface" (#122)

This PR is relatively straightforward. All in all, there were only a couple of editorial changes, all of which in the [*GamepadButton* Interface](https://w3c.github.io/gamepad/#gamepadbutton-interface) section:

- A few missing commas have been added.
- An instance of "value" has been changed to "attribute", because the "`pressed` attribute" had been specified under that nomenclature, and a property shouldn't be able to mirror a value, but rather mirror another property.
- `<a>` has been used to reference the definition of `pressed`.
- There used to be an instance where both "0" and "zero" would be used in the same sentence. Since both referenced the same value, I've replaced "0" with "zero".
- The term "respectively" has been added in a phrase where two numeric values were assigned to two button states in order to avoid confusion.

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The following tasks have been completed:

 * [ ] Modified Web platform tests (link to pull request)

Implementation commitment:

 * [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
 * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
 * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

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

  https://github.com/w3c/gamepad/pull/122

-- Commit Summary --

  * Editorial: Fixed a few inconsistencies in "GamepadButton Interface"

-- File Changes --

    M index.html (26)

-- Patch Links --

https://github.com/w3c/gamepad/pull/122.patch
https://github.com/w3c/gamepad/pull/122.diff

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