Fix documentation inconsistency

[jongillham]

What this PR does / why we need it: It fixes a documentation formatting inconsistency.

Which issue(s) this PR fixes:

Does this PR introduce a user-facing change?:

It changes a documentation subheading weight to be consistent with previous subheadings.

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[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jongillham, woop

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[woop]

Thanks @jongillham