Fix refpurpose of Imagick::getImageAttribute (#807)

Author: zors1

reference/imagick/imagick/getimageattribute.xml

@@ -4,7 +4,7 @@
 <refentry xml:id="imagick.getimageattribute" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
  <refnamediv>
   <refname>Imagick::getImageAttribute</refname>
-  <refpurpose>Description</refpurpose>
+  <refpurpose>Returns a named attribute</refpurpose>
  </refnamediv>
 
  <refsynopsisdiv>