maintaining.html: Mention groff_man_style(7)

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
diff --git a/maintaining.html b/maintaining.html
index 77dcb43..6bd9880 100644
--- a/maintaining.html
+++ b/maintaining.html
@@ -95,6 +95,11 @@
         </li>
         <li>
             <span class="man-page">
+            <a href="https://man7.org/linux/man-pages/man7/groff_man_style.7.html">groff_man_style(7)</a>
+            </span>
+        </li>
+        <li>
+            <span class="man-page">
             <a href="https://man7.org/linux/man-pages/man7/groff_char.7.html">groff_char(7)</a>
             </span>
         </li>