Fix doubleclick rtc example page, allowlist json in RTC macro

[powerivq]

data-json attribute does not exist, so it will simply error out. There are publishers misled by this template. TGT macro which uses the json attribute appears to be the correct one.

[calebcordry]

hmm data-json is explicitly mentioned here https://github.com/ampproject/amphtml/blob/master/extensions/amp-ad-network-doubleclick-impl/doubleclick-rtc.md#available-url-macros are we sure this is the right fix?

[powerivq]

@calebcordry That data-json looks stale. Everything else in the same doc only references to json attribute, and there are no signs of data-json any more. Maybe I should allowlist json instead?

[amp-owners-bot]

Hey @jeffkaufman! These files were changed:

extensions/amp-ad-network-doubleclick-impl/0.1/amp-ad-network-doubleclick-impl.js
extensions/amp-ad-network-doubleclick-impl/doubleclick-rtc.md

[calebcordry]

It also seems to be used in doubleclick auto ads, but not sure if that matters for RTC case

amphtml/extensions/amp-auto-ads/0.1/doubleclick-network-config.js

Line 69 in fac0f5f

'json': this.autoAmpAdsElement_.getAttribute('data-json'),

Maybe @lannka or @zombifier have more context?

[powerivq]

I did a git blame, and it appears data-json was never permitted, contrary to what the doc says. Maybe @keithwrightbos knows more on the history of this?

[powerivq]

Updated the example to use ATTR(json) instead. This way this PR does not change the originally intended behavior.

[calebcordry]

Thanks for looking into this