(BotConfig) add missing getter and setter

src/NadekoBot/Services/Database/Models/BotConfig.cs

@@ -34,7 +34,7 @@ namespace NadekoBot.Services.Database.Models
         public List<EightBallResponse> EightBallResponses { get; set; } = new List<EightBallResponse>();
         public List<RaceAnimal> RaceAnimals { get; set; } = new List<RaceAnimal>();
 
-        public string DMHelpString;
+        public string DMHelpString { get; set; } 
         public string HelpString { get; set; }
 
         public int MigrationVersion { get; set; }