Added programlisting role for syntax highlighting for "coerce into union type" example. (#4901)

Author: mumumu

language/types/type-juggling.xml

@@ -250,7 +250,7 @@
 
    <example>
     <title>Example of types being coerced into a type part of the union</title>
-    <programlisting>
+    <programlisting role="php">
 <![CDATA[
 <?php
 // int|string