Fixing arbitrary spelling of the word arbetrary

Oddly, this is far from the first time a person whose native language
is not english has corrected my spelling.

src/Orchard/Data/Conventions/StringLengthConvention.cs

@@ -5,7 +5,7 @@ using FluentNHibernate.Conventions.Instances;
 namespace Orchard.Data.Conventions {
     public class StringLengthMaxAttribute : StringLengthAttribute {
         public StringLengthMaxAttribute() : base(10000) {
-            // 10000 is an arbetrary number large enough to be in the nvarchar(max) range 
+            // 10000 is an arbitrary number large enough to be in the nvarchar(max) range 
         }
     }