Make h3 smaller than h2
Make h3 smaller than h2

Summary:
After moving to docusaurus v2, our h3 tags are bigger than the h2 tags.
Removing our custom css fixes this, so doing that to get closer to the default theme.

Reviewed By: mweststrate

Differential Revision: D21283840

fbshipit-source-id: ceaf7488b90bbb90a463037d9e1dfcca1d694366

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