Fix broken asset link
Fix broken asset link

Summary:
Assets should never be referenced by /doc/assets, as it breaks docusaurus' baseUrl conversion.

I'd write a lint rule for this but I don't know if it affects docusaurus v2, might be better to just migrate to that.

Reviewed By: passy

Differential Revision: D21153686

fbshipit-source-id: d839015e989a64f897617855ece77439da388cb7

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper