bug in the demosha256 example from fcl-hash

there is a small bug in demosha256 fromthe fcl-hash package. it is a two second fix: the two occurances of sha256hexa need to be replaced with calls to the class procedure Tsha256.DigestHexa. No further change needed.

  aSource:='abc';
  S:=TEncoding.UTF8.GetAnsiBytes(aSource);
  TSHA256.DigestHexa(S, Digest);
  if (Digest<> 'BA7816BF8F01CFEA414140DE5DAE2223B00361A396177A9CB410FF61F20015AD') then
    raise Exception.Create('ERR_SHA256');
  SetLength(S,0);
  TSHA256.DigestHexa(S, Digest);