[docs] update docs for create-dapp command (MystenLabs#15068)

## Description 

We previously documented the command as `pnpm create
@mysten/create-dapp` but should be using `pnpm create @mysten/dapp`
which will work when converted to yarn or npm (the `create-` prefix for
the package name is automatically applied by the `create` command)

## Test Plan 

How did you test the new or updated feature?

---
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### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes