message: fix a typo. this shows up in GraphViewer's tooltip
diff: @@ -1128,7 +1128,7 @@ string HloDotDumper::GetInstructionNodeMetadata(const HloInstruction* instr) {
  }
  if (!instr->metadata().source_file().empty() &&
  instr->metadata().source_line() != 0) {
  - lines.push_back(StrFormat("op_type: %s:%d", instr->metadata().source_file(),
  + lines.push_back(StrFormat("source: %s:%d", instr->metadata().source_file(),
  instr->metadata().source_line()));
  }
  