Update to G131 (#339)

Hi everyone,

On the call today we agreed to update the procedure of G131 based on Jake's comment:
https://github.com/w3c/wcag/issues/339#issuecomment-514129238

But it didn't include a pull request. I've done a PR:
https://github.com/w3c/wcag/pull/830/files

Since we agreed that change I'll merge it, but if anyone has an issue with the exact PR leave a comment and we'll go around the survey-loop again.

Kind regards,

-Alastair

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www.nomensa.com<http://www.nomensa.com/> / @alastc