_content: fix gorebuild import path

Somehow I looked directly at the URL and thought whether it'd be better
for the https://pkg.go.dev/golang.org/x/build/cmd/gorebuild link
to be shortened to https://golang.org/x/build/cmd/gorebuild or not¹,
yet completely failed to notice that it said tools instead of build.
Whoops!

¹ The upsides are that it's shorter and lets the package stay in control
  for deciding what it's "home" is, but the downside is that in practice
  it has no effect and just adds an extra redirect.

Change-Id: I43b02a7eb6405a0b7700c522cdd6a7939bcf9f1c
Reviewed-on: https://go-review.googlesource.com/c/website/+/515240
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

_content/rebuild.html

@@ -13,7 +13,7 @@ pre { margin-left: 2em; }
 <p>
 As of Go 1.21, Go's binary toolchain downloads served by <a href="/dl/">go.dev/dl/</a> can be
 reproduced and verified by anyone, on any platform,
-using <a href="https://pkg.go.dev/golang.org/x/tools/cmd/gorebuild">golang.org/x/tools/cmd/gorebuild</a>.
+using <a href="https://pkg.go.dev/golang.org/x/build/cmd/gorebuild">golang.org/x/build/cmd/gorebuild</a>.
 </p>
 
 <p>