maintaining.html: Use https:// location of Linus's tree (rather than git://)

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
diff --git a/maintaining.html b/maintaining.html
index 86dae4d..e72c235 100644
--- a/maintaining.html
+++ b/maintaining.html
@@ -150,7 +150,7 @@
             You can get a copy of Linus' Git tree (i.e., the current development tree)
             using this command:
             <!-- https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/ -->
-            <pre class="shell">$ <strong>git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git linux</strong></pre>
+            <pre class="shell">$ <strong>git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git</strong></pre>
             Thereafter, just update the copy using <span class="func">git pull</span>.
     </p>