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

`create trigger` commands in [examples/wal_plugin/README.md](https://github.com/influxdata/influxdb3_plugins/blob/main/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 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
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.

Open examples/wal_plugin/README.md and find the `create trigger` examples that use `--plugin-filename`; compare them with the `influxdb3 create trigger --help` output described in the issue. Done means the single-file examples use `--path wal_plugin.py`, with no deprecated flag remaining in those commands.

Written by the indexing model from the issue text.