Update Help.cs (#1968)

Fix Object reference not set to an instance of an object. for shell help command

Author: simonhoss

LiteDB.Shell/Commands/Help.cs

@@ -28,7 +28,7 @@ public void Execute(StringScanner s, Env env)
             // getting all HelpAttributes inside assemblies
             var helps = AppDomain.CurrentDomain.GetAssemblies()
                 .SelectMany(x => x.GetTypes())
-                .Select(x => CustomAttributeExtensions.GetCustomAttributes(typeof(HelpAttribute), true).FirstOrDefault())
+                .Select(x => CustomAttributeExtensions.GetCustomAttributes(x, typeof(HelpAttribute), true).FirstOrDefault())
                 .Where(x => x != null)
                 .Select(x => x as HelpAttribute)
                 .ToArray();
@@ -49,4 +49,4 @@ public void Execute(StringScanner s, Env env)
             }
         }
     }
-}
+}