singleton.xml: remove extra "an" before "enumerations" (#5487)

Author: xabbuh

language/types/singleton.xml

@@ -19,7 +19,7 @@
 
  <note>
   <simpara>
-   It is not possible to define custom singleton types. Consider using an
+   It is not possible to define custom singleton types. Consider using
    <link linkend="language.types.enumerations">enumerations</link> instead.
   </simpara>
  </note>