update helloworld

[sonichen]

Thank you for your amazing work. I have a suggestion to the document.

For Run on Sandbox in HelloWorld.
the cmd

soroban contract invoke \
    --wasm target/wasm32-unknown-unknown/release/[project-name].wasm \
    --id 1 \
    -- \
    hello \
    --to friend

Strictly speaking, it is [project_name].wasm, not [project-name].wasm. For example, project hello-soroban generate hello_soroban.wasm . The [project_name].wasm may bring confusion for beginners.

[ElliotFriend]

That's a really great catch, @sonichen! Thank so much for submitting the PR!