Minor fix for CommandContext

Author: RealMangorage

src/main/java/org/mangorage/mangobotcore/api/command/v1/CommandContext.java

@@ -42,7 +42,7 @@ public <T> T getArgument(Argument<T> argument) {
     }
 
     public <T> T getArgumentOrElse(OptionalArg<T> optionalArgument) {
-        if (index > arguments.length) // End of arguments...
+        if (index >= arguments.length) // End of arguments...
             return optionalArgument.getDefaultValue();
 
         T result = null;