Add missing type name "multirange" in docs chapter title

Discussion: https://postgr.es/m/CAFj8pRDioOxiJgmgw9TqQqZ3CxnJC4P5B2Oospf2eMgAjJuewA%40mail.gmail.com
Author: Pavel Stehule, Alexander Korotkov
Reviewed-by: Justin Pryzby, Tom Lane

doc/src/sgml/func.sgml

@@ -18165,7 +18165,7 @@ SELECT NULLIF(value, '(none)') ...
   </sect1>
 
  <sect1 id="functions-range">
-  <title>Range Functions and Operators</title>
+  <title>Range/Multirange Functions and Operators</title>
 
   <para>
    See <xref linkend="rangetypes"/> for an overview of range types.