Changed the mapping for the Fatal SeriLog event level to map to the Critical Appliation Insights SeverityLevel

Author: Aaron Witt

src/Serilog.Sinks.ApplicationInsights/Sinks/ApplicationInsights/LogEventLevelExtensions.cs

@@ -39,8 +39,9 @@ public static class LogEventLevelExtensions
                 case LogEventLevel.Warning:
                     return SeverityLevel.Warning;
                 case LogEventLevel.Error:
-                case LogEventLevel.Fatal:
                     return SeverityLevel.Error;
+                case LogEventLevel.Fatal:
+                    return SeverityLevel.Critical;
             }
 
             return null;