fix filtering level ovverrides

cfg.Key != "Default" instead of cfg.Value != "Default"

Author: micdenny

src/Serilog.Extensions.Logging.File/Microsoft/Extensions/Logging/FileLoggerExtensions.cs

@@ -73,7 +73,7 @@ public static ILoggerFactory AddFile(this ILoggerFactory loggerFactory, IConfigu
             }
 
             var levelOverrides = new Dictionary<string, LogLevel>();
-            foreach (var overr in levelSection.GetChildren().Where(cfg => cfg.Value != "Default"))
+            foreach (var overr in levelSection.GetChildren().Where(cfg => cfg.Key != "Default"))
             {
                 LogLevel value;
                 if (!Enum.TryParse(overr.Value, out value))