Fix auto-detect save file on load

introduced in
99673706a0
reference the field that stores the full file path

https://projectpokemon.org/home/forums/topic/46293-not-reading-card/

PKHeX.WinForms/MainWindow/Main.cs

@@ -1181,7 +1181,7 @@ private static bool DetectSaveFile(out string path)
             if (sav == null && !string.IsNullOrWhiteSpace(msg))
                 WinFormsUtil.Error(msg);
 
-            path = sav?.FileName;
+            path = sav?.FilePath;
             return path != null && File.Exists(path);
         }