Update common_configs.rb index date format

[bukovyn]

Update comment for index date format

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
86a8aa9

Please, read and sign the above mentioned agreement if you want to contribute to this project

[robbavey]

Hi @bukovyn Would you mind signing the CLA? We need this to be signed before we can merge your commit.

Thanks!