Docs: Fix duplicate 'type' key in Nessie Flink Python example

[thswlsqls]

Closes #17089

Summary

• The Flink Python API CREATE CATALOG example set the 'type' key twice in the same properties map ('iceberg' then 'nessie').
• The repo's established convention (flink.md) is 'type'='iceberg' plus a separate 'catalog-type'='<impl>' key to select the catalog implementation.
• Changed the second 'type'='nessie' to 'catalog-type'='nessie'. A prior fix attempt, PR Docs: fix create nessie catalog in Flink #12978, proposed the same change but was closed by stale-bot with no merit objection; this bug is still present on main.

Testing done

• Docs-only change, no behavior change — no test added. Verified against docs/docs/flink.md line 189-197's type/catalog-type convention.

[ebyhr]

Does this example really work? I think Flink only supports hadoop, hive, rest in catalog-type property.

iceberg/flink/v2.1/flink/src/main/java/org/apache/iceberg/flink/FlinkCatalogFactory.java

Lines 101 to 120 in 3038fde

	String catalogType = properties.getOrDefault(ICEBERG_CATALOG_TYPE, ICEBERG_CATALOG_TYPE_HIVE);
	switch (catalogType.toLowerCase(Locale.ENGLISH)) {
	case ICEBERG_CATALOG_TYPE_HIVE:
	// The values of properties 'uri', 'warehouse', 'hive-conf-dir' are allowed to be null, in
	// that case it will
	// fallback to parse those values from hadoop configuration which is loaded from classpath.
	String hiveConfDir = properties.get(HIVE_CONF_DIR);
	String hadoopConfDir = properties.get(HADOOP_CONF_DIR);
	Configuration newHadoopConf = mergeHiveConf(hadoopConf, hiveConfDir, hadoopConfDir);
	return CatalogLoader.hive(name, newHadoopConf, properties);
	case ICEBERG_CATALOG_TYPE_HADOOP:
	return CatalogLoader.hadoop(name, hadoopConf, properties);
	case ICEBERG_CATALOG_TYPE_REST:
	return CatalogLoader.rest(name, hadoopConf, properties);
	default:
	throw new UnsupportedOperationException(
	"Unknown catalog-type: " + catalogType + " (Must be 'hive', 'hadoop' or 'rest')");