fix prdoc cmd

[gui1117]

The command no longer support multiple audience, also the mapping was wrong.

[mordamax]

/cmd prdoc --help --clean

[github-actions]

Command help:

usage: /cmd prdoc [-h] [--pr PR]
                  [--audience [{runtime_dev,runtime_user,node_dev,node_operator} ...]]
                  [--bump {patch,minor,major,silent,ignore,none}] [--force]

options:
  -h, --help            show this help message and exit
  --pr PR               The PR number to generate the PrDoc for.
  --audience [{runtime_dev,runtime_user,node_dev,node_operator} ...]
                        The audience of whom the changes may concern. Example:
                        --audience runtime_dev node_dev
  --bump {patch,minor,major,silent,ignore,none}
                        A default bump level for all crates. Example: --bump
                        patch
  --force               Whether to overwrite any existing PrDoc.