plugin_config_formatter: update new doc URL

[nokute78]

Signed-off-by: Takahiro YAMASHITA [email protected]

Which issue(s) this PR fixes:
None

What this PR does / why we need it:
fluent-plugin-config-format outputs old style URLs. (not gitbook style)

This PR is to make fluent-plugin-config-format supporting gitbook style URLs.

Old output is

[taka@localhost fluentd]$ bundle exec bin/fluent-plugin-config-format input syslog
## Plugin helpers

* [parser](https://docs.fluentd.org/v1.0/articles/api-plugin-helper-parser)
* [compat_parameters](https://docs.fluentd.org/v1.0/articles/api-plugin-helper-compat_parameters)
* [server](https://docs.fluentd.org/v1.0/articles/api-plugin-helper-server)

* See also: [Input Plugin Overview](https://docs.fluentd.org/v1.0/articles/input-plugin-overview)

New output is

[taka@localhost fluentd]$ bundle exec bin/fluent-plugin-config-format input syslog
## Plugin helpers

* [parser](https://docs.fluentd.org/v/1.0/plugin-helper-overview/api-plugin-helper-parser)
* [compat_parameters](https://docs.fluentd.org/v/1.0/plugin-helper-overview/api-plugin-helper-compat_parameters)
* [server](https://docs.fluentd.org/v/1.0/plugin-helper-overview/api-plugin-helper-server)

* See also: [Input Plugin Overview](https://docs.fluentd.org/v/1.0/input#overview)

Docs Changes:
None

Release Note:
plugin_config_formatter: support gitbook URLs.

[repeatedly]

Thanks!