[streams] Typo in ErrorReadableStream algorithm (#387)

In the HTML draft currently available at https://streams.spec.whatwg.org/#error-readable-stream, step 6 is probably wrong. It should probably say something like: if reader is undefined, return undefined.
Step 6a could also be renumbered as step 7.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/387