_content: fix Markdown link in rebuild.html

This page has .html extension, not .md, so use an HTML link.

Change-Id: Ifd5646c8b2875c805ce611ea8e2885b89136223f
Reviewed-on: https://go-review.googlesource.com/c/website/+/546695
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

_content/rebuild.html

@@ -15,7 +15,7 @@ As of Go 1.21, Go's binary toolchain downloads served by <a href="/dl/">go.dev/d
 reproduced and verified by anyone, on any platform,
 using <a href="https://pkg.go.dev/golang.org/x/build/cmd/gorebuild">golang.org/x/build/cmd/gorebuild</a>.
 For more details, see the blog post
-“[Perfectly Reproducible, Verified Go Toolchains](/blog/rebuild).”
+“<a href="/blog/rebuild">Perfectly Reproducible, Verified Go Toolchains</a>.”
 </p>
 
 <p>