move ref DirectoryEntryConverter to System.DirectoryServices.Design n…

…amespace (dotnet#75235)

The DirectoryEntryConverter's implementation is in the System.DirectoryServices.Design namespace. However, the reference was mistakenly put in the System.DirectoryServices namespace. This was flagged by the new ApiCompat tool, where the type argument to the TypeConverter attribute was different (since they're in different namespaces).

src/libraries/System.DirectoryServices/ref/System.DirectoryServices.manual.cs

@@ -4,9 +4,15 @@
 // Changes to this file must follow the https://aka.ms/api-review process.
 // ------------------------------------------------------------------------------
 
+using System.DirectoryServices.Design;
+
 namespace System.DirectoryServices
 {
     [System.ComponentModel.TypeConverter(typeof(DirectoryEntryConverter))]
     public partial class DirectoryEntry { }
-    internal sealed class DirectoryEntryConverter { }
 }
+
+namespace System.DirectoryServices.Design
+{
+    internal sealed class DirectoryEntryConverter { }
+}