fix: markdown mistake when referring to an AIP

[MasterMedo]

Currently, the AIP reference renders wrongly, and this commit fixes it.

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[andrei-scripniciuc]

thanks for sending this out! the intent here was to link to the AIP, but it seems we missed including the link at the bottom of the page. Could you instead add a reference link to https://google.aip.dev/142? Something like:

[AIP-142]: https://google.aip.dev/142

https://www.markdownguide.org/basic-syntax/#reference-style-links

[MasterMedo]

Sorry for the slow response.

the intent here was to link to the AIP, but it seems we missed including the link at the bottom of the page.

I don't think that needs to be added for the link to work. Look at other AIPs at https://google.aip.dev/162, they are automatically linked to the appropriate AIP.