Use 'documents' as the DID genesis JSON key, instead of 'Documents'

[youngjoon-lee]

Previously, the genesis.json looked like:

{
  "app_state": {
    "did": {
      "Documents": {
      }
  }
}

Let's use documents instead of Documents.

```json
{
  "app_state": {
    "did": {
      "documents": {
          "did:panacea:9iofqBjmEkL4XRiCguNxun3GVKn2Ghoqud39uS25S1aY": {
          }
      }
  }
}

[cl9200]

👍