Update new.rs (MystenLabs#17828)

Edited for clarity.

## Description 

Saw in a merged PR. The use of "please" is not advised as it suggests
package naming rules are optional.

## Test plan 

How did you test the new or updated feature?

---

## 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:

external-crates/move/crates/move-cli/src/base/new.rs

@@ -52,8 +52,8 @@ impl New {
 
         if !Identifier::is_valid(&name) {
             return Err(anyhow!(
-                "Invalid package name, please use lowercase letters, numbers, and underscores, \
-                 should start with a lowercase letter"
+                "Invalid package name. Package name must start with a lowercase letter \
+                 and consist only of lowercase letters, numbers, and underscores."
             ));
         }