DocBook/drm: Insert missing `an'

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

Documentation/DocBook/drm.tmpl

@@ -342,7 +342,7 @@
 	DRM_WRITEn() macros to access the registers on your device, or
 	use driver specific versions to offset into your MMIO space
 	relative to a driver specific base pointer (see I915_READ for
-	example).
+	an example).
       </para>
       <para>
 	If your device supports interrupt generation, you may want to