static/frontend/styleguide: fix styleguide title class
static/frontend/styleguide: fix styleguide title class

The class removed is already applied one level up in
the DOM tree.

Change-Id: I46543531a49618c30e2f5da2ed2f8b29cfcc4b5e
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/335495
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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