Update array-unique.xml Add the missing comma

Author: mmalferov

reference/array/functions/array-unique.xml

@@ -25,7 +25,7 @@
   <note>
    <simpara>
     Two elements are considered equal if and only if
-    <literal>(string) $elem1 === (string) $elem2</literal> i.e. 
+    <literal>(string) $elem1 === (string) $elem2</literal>, i.e. 
     when the string representation is the same, the first element will be used.
    </simpara>
   </note>