content/static: remove + symbol from popular search terms link
content/static: remove + symbol from popular search terms link

Fixes a typo in the popular search term links.

Change-Id: Idce1750fafd8387dab2dd9084e15d8b9b087468a
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/245881
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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