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

[thswlsqls]

Apache Iceberg version
main @ 49b89a8

Query engine
Flink

Please describe the bug
The Flink Python API CREATE CATALOG example in docs/docs/nessie.md (lines 94-99) sets the 'type' key twice in the same properties map: 'type'='iceberg', 'type'='nessie', .... The repo's established convention for selecting a catalog implementation (docs/docs/flink.md lines 189-197) is 'type'='iceberg' (fixed Flink catalog factory identifier) plus a separate 'catalog-type'='<impl>' key. nessie.md's example is the only one in the repo that duplicates type instead of using catalog-type. A prior fix attempt, PR #12978, proposed exactly this correction but was closed by stale-bot with no merit objection ("This is not a judgement on the merit of the PR"); the bug is still present on main.

Steps to reproduce
Copy the Python snippet from nessie.md line 94-99 and run it against a Flink Table Environment: the duplicate 'type' key means no catalog-type is ever set, so a Nessie catalog is not created as intended.

Additional context
N/A.