Fix grammar in filter_has_var description (#4684)

Author: Maddily

reference/filter/functions/filter-has-var.xml

@@ -4,7 +4,7 @@
 <refentry xml:id="function.filter-has-var" xmlns="http://docbook.org/ns/docbook">
  <refnamediv>
   <refname>filter_has_var</refname>
-  <refpurpose>Checks if variable of specified type exists</refpurpose>
+  <refpurpose>Checks if a variable of the specified type exists</refpurpose>
  </refnamediv>
  <refsect1 role="description">
   &reftitle.description;