static/frontend: do not drop URL query when expanding README

Fixes golang/go#77164

Change-Id: Ic6f1097777704447c2689d44609313912c3fb9c9
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/736120
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ethan Lee <ethanalee@google.com>

Author: nicholashusin