Outdated instructions for wasm

[xJonathanLEI]

The --export flag has been removed, but the wasm guide still tells users to use it.

criterion.rs/book/src/user_guide/wasi.md

Lines 85 to 91 in 326ebbb

	### Exporting results
	Writing benchmark results to an in-memory filesystem in the browser is not very useful on its own. Luckily the results are easy to export and download as JSON:
	```properties
	hex --bench --export=base | download
	```

The book needs to be updated with the latest instructions.

And... as a side question, how are we supposed to export wasm results now that the flag is gone (can't build cargo-criterion for wasm)? Thanks in advance.