webxr-samples: use "index.html" suffix for links

This helps ensure that the links work even on webservers that don't
auto-redirect directories to "index.html".

Automatically changed via:

$ perl -pi -e \
  's{href=(['\''"])([^:]*?)/(['\''"])}{href=$1$2/index.html$3}' \
   *.html */*.html

Change-Id: Idc6b73e8917a7b98887978d684481888d8e8b95f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627750
Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662783}
26 files changed