Before this change, the example config in some places had
`admin_value_regex` and `admin_group_regex` as list items under
`admin_mapping`, which caused the following error:
```
panic: failed to load config from yaml: yaml error: yaml: unmarshal errors:
line 45: cannot unmarshal !!seq into config.OauthAdminMapping
```
Removing the list and making them normal YML items prevents this error,
as `admin_mapping` does not expect a list.
This also fits my understanding of the [config implementation](7557a6ef5a/internal/config/auth.go (L32))
and another [example config file](662e9c0549/config.yml.sample (L88)).
Signed-off-by: klmmr <35450576+klmmr@users.noreply.github.com>
