docs(a11y): fix tertiary nav heading color contrast

[dancormier]

This is a minor accessibility fix for our Stacks docs to change the tertiary nav (right side) heading color from fc-theme-primary to black-400.

See https://stackexchange.slack.com/archives/C27RWNQN9/p1716992049112259?thread_ts=1716979306.515569&cid=C27RWNQN9

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