docs: example requires example feature to run

[azzamsa]

Description

You will get this error without passing --features=examples.

🐡 ❯ cargo run --example client
error: target `client` in package `iroh` requires the features: `examples`
Consider enabling them by passing, e.g., `--features="examples"

Breaking Changes

No.

Notes & open questions

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.
• All breaking changes documented.

Closes #2431

[flub]

thanks!

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