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

Already done for update-man.cmake.
diff --git a/man/zip_file_strerror.html b/man/zip_file_strerror.html
index fe81f1c..019efe0 100644
--- a/man/zip_file_strerror.html
+++ b/man/zip_file_strerror.html
@@ -108,7 +108,7 @@
 <table class="foot">
   <tr>
     <td class="foot-date">December 18, 2017</td>
-    <td class="foot-os">NetBSD 9.99.67</td>
+    <td class="foot-os">NiH</td>
   </tr>
 </table>
 </body>