fix typo in "nitpick_choice_spec" attribute name (singular, not plural)

--- a/src/HOL/HOL.thy	Wed Mar 17 09:14:43 2010 +0100
+++ b/src/HOL/HOL.thy	Wed Mar 17 12:01:01 2010 +0100
@@ -2022,7 +2022,7 @@
 )
 structure Nitpick_Choice_Specs = Named_Thms
 (
-  val name = "nitpick_choice_specs"
+  val name = "nitpick_choice_spec"
   val description = "choice specification of constants as needed by Nitpick"
 )
 *}