Fix Natspec model - method key number -> string

[loredanacirstea]

@model()
export class NatspecMethods extends Model {
  [prop: number]: NatspecMethod;

  constructor(data?: Partial<NatspecMethods>) {
    super(data);
  }
}

https://github.com/loredanacirstea/dType-pipeos-contracts/commit/04813ea289fb43a693d758b5eb2d69863be195b6#diff-2b290993f649c2c4d2dcb4d6bf564c33R45

For some reason, string was not allowed, even though in theory, number and string are the only ones allowed.