fix: docs/example

[kristian-nygaard-jensen]

fs.add is not a function
"Invalid key type" key type must be specified

https://github.com/libp2p/js-libp2p/blob/main/packages/keychain/src/keychain.ts#L157

fix: docs/example

Description

Two function calls need to be modified for the examples to work.

Notes & open questions

I will try to write tests that prove the issues

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[achingbrain]

Thanks for opening this - can you please swap refs to RSA keys to use Ed25519 keys instead?

[SgtPooki]

since the changes are not made to the index.ts files, they will be overwritten. can you make the modifications to the index.ts files so our doc gen updates the readme.md?