[whatwg/streams] Fix readable/writable file stream examples (PR #1191)

These examples weren't closing the filehandle correctly (they called `close` on a nonexistent `fd` variable, rather than calling the `fileHandle.close` method).

Also the ReadableStream example misused the `fileHandle.read` call: it was supplying an ArrayBuffer and v's byteOffset within the ArrayBuffer, but the API requires supplying v itself and an offset relative to v's start.

Also added the file offset `position` to that `fileHandle.read` call. There's no reason otherwise for the example to keep track of this. Alternatively, the argument could be supplied as null, and we just rely on implicit file positioning.

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(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)

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

  https://github.com/whatwg/streams/pull/1191


-- Commit Summary --

  * Fix readable/writable file stream examples

-- File Changes --

    M index.bs (9)

-- Patch Links --

https://github.com/whatwg/streams/pull/1191.patch

https://github.com/whatwg/streams/pull/1191.diff


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