docs: fix incorrect path to Express server example

[reallesee]

Fixed reference to Express server example in main README.The comment incorrectly pointed to examples/typescript/servers/express.tswhich doesn't exist. Updated to correct path: examples/typescript/servers/express/index.ts

[cb-heimdall]

✅ Heimdall Review Status

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[vercel[bot]]

@reallesee is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.