docs/website/docs.html: use HTTPS for nightly.buildroot.org links

Browsers nowadays complain about HTTP downloads (E.G. manual.pdf) from a
site served over HTTPS, so also use HTTPS for the nightly.buildroot.org
manual links.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/website/docs.html

@@ -26,7 +26,7 @@
 
 	  <h3>HTML <br>
 	    <a href="/manual.html">Stable</a> |
-	    <a href="http://nightly.buildroot.org/manual.html">Nightly</a>
+	    <a href="https://nightly.buildroot.org/manual.html">Nightly</a>
 	  </h3>
 
 	</div>
@@ -45,7 +45,7 @@
 
 	  <h3>PDF <br>
 	    <a href="/downloads/manual/manual.pdf">Stable</a> |
-	    <a href="http://nightly.buildroot.org/manual.pdf">Nightly</a>
+	    <a href="https://nightly.buildroot.org/manual.pdf">Nightly</a>
 	  </h3>
 
 	</div>
@@ -64,7 +64,7 @@
 
 	  <h3>ASCII <br>
 	    <a href="/downloads/manual/manual.text">Stable</a> |
-	    <a href="http://nightly.buildroot.org/manual.text">Nightly</a>
+	    <a href="https://nightly.buildroot.org/manual.text">Nightly</a>
 	  </h3>
 
 	</div>