Minor amp-ad doc update about amp-ad script

[jasti]

Fixes #4140.

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[jasti]

No, googleBOT. It's not and I've signed it.

[zhouyx]

LGTM, And good job on complaining to googleBOT!

[jasti]

@zhouyx Can you please merge this? I don't have auth to do it.

[lannka]

@jasti CLA didn't pass. I met the same issue when I signed using my Google corp account. Then I signed again with my personal gmail, it just worked.

[jasti]

Signed CLA with my personal email too (and added to GH).

[googlebot]

CLAs look good, thanks!