Fix the allowed attributes for amp-position-observer

[felixarntz]

I tried to use amp-position-observer with the plugin earlier and noticed that it strips out the target-id attribute as invalid. That is because the whitelist for that tag only allows a target attribute which appears to be wrong to me. The AMP documentation states that target-id is the correct name of the attribute.

[westonruter]

Thanks for pointing this out. In this case it means it is time to do another update of the generated PHP from the spec. Previous PRs where this was done:

• Run spec update to add support for amp-mathml, amp-riddle-quiz, amp-wistia-player, amp-byside-content, amp-bind-macro #943
• Add comment sorting support #1024
• Update generated sanitizer (spec file revision 595) #1057
• Update spec generated from amphtml to file revision 642 (commit 3953b25) #1172

Here are some instructions: https://github.com/Automattic/amp-wp/blob/develop/contributing.md#updating-allowed-tags-and-attributes

[westonruter]

I'll try to re-generating from the spec by tomorrow unless you beat me to it.

[westonruter]

It turns out that target-id is actually incorrect in the docs:

It is supposed to be just target: https://github.com/ampproject/amphtml/blob/1143ef048f4845ff0ae21a78c07053646508fc36/extensions/amp-position-observer/validator-amp-position-observer.protoascii#L45

[westonruter]

I've opened a PR to fix the docs: ampproject/amphtml#17272