Documentation: incorrect configuration setup

__Steps to reproduce:__
For InfluxDB v2.x

1. Browse to https://docs.influxdata.com/influxdb/v2.1/reference/config-options/
2. Note the text "export INFLUXD_CONFIG_PATH=/path/to/custom/config/directory"
3. Text reads:
```
To customize the directory path of the configuration file, set the INFLUXD_CONFIG_PATH environment variable to your custom path.

export INFLUXD_CONFIG_PATH=/path/to/custom/config/directory

On startup, influxd will check for a config.* in the INFLUXD_CONFIG_PATH directory.
```

__Expected behavior:__
The text should read:
```
To customize the path to the configuration file, set the INFLUXD_CONFIG_PATH environment variable to point to your configuration file.

export INFLUXD_CONFIG_PATH=/path/to/custom/config/directory/config.(yaml|toml|json)

On startup, influxd will read the file specified in the INFLUXD_CONFIG_PATH variable.
```

__Actual behavior:__
If you follow the instructions in the documentation, your influxdb instance will not read your configuration file.

__Environment info:__

* `Linux 5.16.9-200.fc35.x86_64 x86_64`
* `InfluxDB 2.1.1 (git: 657e1839de) build_date: 2021-11-09T03:03:48Z`

Start with the configuration options page at https://docs.influxdata.com/influxdb/v2.1/reference/config-options/ and verify how INFLUXD_CONFIG_PATH is consumed by influxd. Update the setup text so it points to a configuration file rather than a directory, and confirm the final wording matches the documented startup behavior.

Written by the indexing model from the issue text.