Rename pullrequest to dev.knative.github.pullrequest in Github sample

[tanzeeb]

As of #340 , the pullrequest event is now dev.knative.github.pullrequest. This PR updates the github sample to reflect the change.

[knative-prow-robot]

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[tanzeeb]

/assign @inlined @mattmoor

[evankanderson]

I believe that @n3wscott is in the process of moving this sample to `knative/docs`, possibly http://github.com/knative/docs/pull/339

…

On Fri, Aug 24, 2018 at 2:45 PM Tanzeeb Khalili ***@***.***> wrote: /assign @inlined <https://github.com/inlined> @mattmoor <https://github.com/mattmoor> — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#395 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AHlyN3U-jRVzaJijEWQtiUjYTdj7fECjks5uUHQCgaJpZM4WMCHz> .

-- Evan Anderson <argent@google.com>

[tanzeeb]

Thanks for the heads up @evankanderson. Looks like the other PR has the changes addressed here, closing.