hydra: bordeaux: Add a how to code snippet.

* hydra/nginx/html/bordeaux/index.html: (Using these substitutes):
Fixed a typo, and added a how to code snippet for users of guix system.

Signed-off-by: Christopher Baines <mail@cbaines.net>

hydra/nginx/html/bordeaux/index.html

@@ -36,7 +36,7 @@
       <b>arm-linux-gnueabihf</b> availability is limited.
     </p>
 
-    <h2>Using these substiutes</h2>
+    <h2>Using these substitutes</h2>
     <p>
       The URL to use to fetch substitutes is <code>https://bordeaux.guix.gnu.org</code>,
       and they're signed with the following key.
@@ -47,5 +47,45 @@
   )
  )</pre>
     </p>
+
+    <h3>On a Guix System</h3>
+    <p>
+      Users of Guix System can start using these substitutes with a
+      bit of scheme code that looks like:
+
+      <pre>(define %my-desktop-services
+  (modify-services
+      %desktop-services
+    (guix-service-type
+     config =>
+     (guix-configuration
+      (inherit config)
+      (substitute-urls
+       (append (list "https://bordeaux.guix.gnu.org/")
+               %default-substitute-urls))
+      (authorized-keys
+       (append
+        (list
+         (plain-file
+                     "bordeaux.guix.gnu.org.signing.key"
+                     "
+(public-key
+ (ecc
+  (curve Ed25519)
+  (q #7D602902D3A2DBB83F8A0FB98602A754C5493B0B778C8D1DD4E0F41DE14DE34F#)
+  )
+ )"))
+        %default-authorized-guix-keys)))))))
+
+
+(operating-system
+    ...
+    (services (append (list (service dhcp-client-service-type)
+                            (service openssh-service-type
+                                     (openssh-configuration
+                                      (openssh openssh-sans-x)
+                                      (port-number 2222))))
+                      %my-desktop-services)))</pre>
+    </p>
   </body>
 </html>