DocBook/drm: a -> an

Signed-off-by: Michael Witten <mfwitten@gmail.com>

Documentation/DocBook/drm.tmpl

@@ -203,7 +203,7 @@
 	<term>DRIVER_HAVE_IRQ</term><term>DRIVER_IRQ_SHARED</term>
 	<listitem>
 	  <para>
-	    DRIVER_HAVE_IRQ indicates whether the driver has a IRQ
+	    DRIVER_HAVE_IRQ indicates whether the driver has an IRQ
 	    handler, DRIVER_IRQ_SHARED indicates whether the device &amp;
 	    handler support shared IRQs (note that this is required of
 	    PCI drivers).