fix netlify CMS configuration

[HugoLiconV]

There is an error when you try to access the CMS.

Error loading the CMS configuration
Config Errors:
'collections[1].fields[1].pattern' should NOT have fewer than 2 items
Check your config.yml file.

This error is because the pattern config is expecting two array items, the regex and error message.

Steps to reproduce

1. gatsby new gatsby-starter-delog https://github.com/W3Layouts/gatsby-starter-delog
2. cd gatsby-starter-delog
3. gatsby develop
4. Go to http://localhost:8000/admin/#/