Use underscored (cookiecutter.underscored) for `PLUGINS` configuration in README.md

[pheus]

cookiecutter-netbox-plugin version

main (d920039)

Python version

3.12

Steps to Reproduce

The cookiecutter-netbox-plugin generates a README.md with the hyphenated project name for PLUGINS and PLUGINS_CONFIG.

1. Run cookiecutter https://github.com/netbox-community/cookiecutter-netbox-plugin.git
2. Fill out the default values

❯ cookiecutter https://github.com/netbox-community/cookiecutter-netbox-plugin.git
  [1/10] plugin_name (ACL):
  [2/10] project_name (NetBox ACL Plugin):
  [3/10] hyphenated (netbox-acl-plugin):
  [4/10] underscored (netbox_acl_plugin):
  [5/10] project_short_description (NetBox plugin for ACL.):
  [6/10] full_name (John Doe):
  [7/10] email ():
  [8/10] github_username ():
  [9/10] version (0.1.0):
  [10/10] Select open_source_license
    1 - Apache-2.0
    2 - MIT
    3 - BSD
    4 - ISC
    5 - GPL-3.0-only
    6 - Not open source
    Choose from [1/2/3/4/5/6] (1): 

3. Check generated README.md

Expected Behavior

Enable the plugin in `/opt/netbox/netbox/netbox/configuration.py`,
 or if you use netbox-docker, your `/configuration/plugins.py` file :

```python
PLUGINS = [
    'netbox_acl_plugin'
]

PLUGINS_CONFIG = {
    "netbox_acl_plugin": {},
}
```

Observed Behavior

(Snippet from the generated README.md with default values)

Enable the plugin in `/opt/netbox/netbox/netbox/configuration.py`,
 or if you use netbox-docker, your `/configuration/plugins.py` file :

```python
PLUGINS = [
    'netbox-acl-plugin'
]

PLUGINS_CONFIG = {
    "netbox-acl-plugin": {},
}
```