Update outdated instruction

[yuma-m]

Issue #, if available: N/A

Description of changes: We have to add -f to specify the file path for kvs_gstreamer_audio_video_sample.

amazon-kinesis-video-streams-producer-sdk-cpp/samples/kvs_gstreamer_audio_video_sample.cpp

Lines 1037 to 1062 in 70f74f1

	if (!STRCMP(argv[i], "-f")) {
	argFlags = 1;
	}
	else if (!STRCMP(argv[i], "-e")) {
	argFlags = 2;
	}
	else {
	continue;
	}
	i++; // found flag, next argument
	switch(argFlags) {
	case 1: //-f
	{
	file_path = string(argv[i]);
	// file path should be at least 4 char (shortest example: a.ts)
	if (file_path.size() < 4) {
	LOG_ERROR("Invalid file path");
	return 1;
	}
	FileInfo fileInfo;
	fileInfo.path = file_path;
	data.file_list.push_back(fileInfo);
	data.uploading_file = true;
	break;
	}

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