[banshee] Banshee.Services: Fix typo from recent refactoring

[AndrÃs Aragoneses <aaragoneses src gnome org>]

commit 949a6510e8c70a1dc8595d1137f5d1538d67a70a
Author: AndrÃs G. Aragoneses <knocte gmail com>
Date:   Mon Sep 19 22:30:52 2011 +0100

    Banshee.Services: Fix typo from recent refactoring
    
    Fix the throw statement for ArgumentNullException.

 .../Banshee.Services/Banshee.Query/BansheeQuery.cs |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/Core/Banshee.Services/Banshee.Query/BansheeQuery.cs b/src/Core/Banshee.Services/Banshee.Query/BansheeQuery.cs
index bb295e4..783faa3 100644
--- a/src/Core/Banshee.Services/Banshee.Query/BansheeQuery.cs
+++ b/src/Core/Banshee.Services/Banshee.Query/BansheeQuery.cs
@@ -399,7 +399,7 @@ namespace Banshee.Query
         public static string GetSort (QueryField field, bool asc)
         {
             if (field == null) {
-                throw new ArgumentNullException ("key");
+                throw new ArgumentNullException ("field");
             }
 
             bool sort_by_year = Banshee.Configuration.Schema.LibrarySchema.SortByAlbumYear.Get ();