Update `wallet_addEthereumChain` Reference (Playground) Example

[httpJunkie]

The example for wallet_addEthereumChain currently gives the user warnings:

This example appears to have the correct parameters according to this link:
https://docs.gnosischain.com/tools/wallets/metamask/

That is why I have not simply submitted a PR with the correct parameters.
It would seem that whatever logic we are using to check:

• The network name matches the chain ID
• The submitted currency symbol matches what we expect for this chain ID
• And that our records for the the submitted RPC URL matches a known provider

Is out of date or incorrect per Gnosis chain docs. We need to check the information we have stored and ensure it matches. But I am not sure which repo and PR this issue should be associated with.

[alexandratran]

According to the Gnosis docs, looks like we should update the example RPC URL to https://rpc.gnosischain.com and symbol to XDAI. @joaniefromtheblock can you address this in your api-specs PR updating the examples?

[joaniekube]

Closed with MetaMask/api-specs#207