update-html.cmake: use NiH as operating system value

Already done for update-man.cmake.
diff --git a/man/zip_error_set.html b/man/zip_error_set.html
index 5e56802..b9bd08f 100644
--- a/man/zip_error_set.html
+++ b/man/zip_error_set.html
@@ -94,7 +94,7 @@
 <table class="foot">
   <tr>
     <td class="foot-date">April 2, 2020</td>
-    <td class="foot-os">NetBSD 9.99.67</td>
+    <td class="foot-os">NiH</td>
   </tr>
 </table>
 </body>