Incorrect environment is specified as the default--the default env
installed if you hit 'enter' is Testnet, not Devnet. This is according
to my own Sui installation on WSL/Ubuntu, and is also described as
"Testnet" in the code snippet immediately above the change.

## Description 
Change "Devnet" to "Testnet"

## Test plan 

How did you test the new or updated feature?
see screenshot for proof that the default is Testnet, not Devnet.

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK: 

<img width="1014" alt="image"
src="https://github.com/MystenLabs/sui/assets/38071276/807347d2-9821-46f0-aab6-0b41f1362e28">