MixHash is missing from Block JSON Schema

[shanejonas]

Describe the bug
MixHash is missing from Block

To Reproduce
Steps to reproduce the behavior:

1. Go to https://playground.open-rpc.org/?schemaUrl=https://services.jade.builders/core-geth/mainnet/1.11.16
2. Click oneth_getBlockByNumber
3. Scroll down and click 'try it now'
4. Enter this in the Inspector Request editor:

{
    "jsonrpc": "2.0",
    "method": "eth_getBlockByNumber",
    "params": [
        "0x9a5883",
        true
    ],
    "id": 0
}

5. MixHash is returned, but is not defined in the openrpc document
6. See screenshots

Screenshots