Deprecated `--plugin-filename` used in examples/wal_plugin

[jstirnaman]

create trigger commands in examples/wal_plugin/README.md use the deprecated --plugin-filename flag.
Because wal_plugin is single-file, we should update the examples to use --path wal_plugin.py.

With influxdb3 v3.8.0, influxdb3 create trigger --help says:

...
      --path <PATH>
          Path to plugin file or directory. For single-file plugins, provide the .py file path. For multi-file plugins, provide the directory path (must contain __init__.py). When not using --upload, this should be the filename/path on the server's plugin-dir. Supports gh: prefix for [influxdb3_plugins](https://github.com/influxdata/influxdb3_plugins) repo
      --plugin-filename <PLUGIN_FILENAME>
          Deprecated: Use --path instead. Python file name of the file on the server's plugin-dir
...

If we use --plugin-filename, we should provide only the filename, not relative or absolute paths.