content/static: tighten spacing below header on unit page
content/static: tighten spacing below header on unit page

Fixes golang/go#42490

Change-Id: Ie3afa8dde6a6a3df58cb6a420d7504ec4facb4b2
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/269557
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite