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

• [x] I have performed a self-review of my own code
• [x] 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