Correct reset-dirs in DTD

Empty elements need to be declared as such in well-formed DTDs.
diff --git a/fonts.dtd b/fonts.dtd
index f8c9f2c..40ecb4e 100644
--- a/fonts.dtd
+++ b/fonts.dtd
@@ -124,7 +124,7 @@
 <!--
     Reset the list of fonts directories
 -->
-<!ELEMENT reset-dirs >
+<!ELEMENT reset-dirs EMPTY>
 
 <!--
     Periodically rescan the font configuration and