Update db_parameter_group.html.markdown

[Ninir]

Fixes #522

[radeksimko]

Thanks @Ninir, this is useful - just one nit - would you mind adding a note about full list of those params?

e.g.

Full list of all parameters can be discovered via [`aws rds describe-db-cluster-parameters`](https://docs.aws.amazon.com/cli/latest/reference/rds/describe-db-cluster-parameters.html) after initial creation of the group.

and

Full list of all parameters can be discovered via [`aws rds describe-db-parameters`](https://docs.aws.amazon.com/cli/latest/reference/rds/describe-db-parameters.html) after initial creation of the group.

[Ninir]

@radeksimko Done! tried to find any online referential for that, no luck :(
Good idea to make use of the CLI there 👍

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!