Fixes off-by-one error when checking for missing arguments.
Before this, running the example gave this scary looking error message:

```
$ cargo run --bin wgpu-examples hello_compute
    Finished dev [unoptimized + debuginfo] target(s) in 0.13s
     Running `target/debug/wgpu-examples hello_compute`
[2023-12-27T22:14:26Z ERROR wgpu::backend::direct] Handling wgpu errors as fatal by default
thread 'main' panicked at 'wgpu error: Validation Error

Caused by:
    In Device::create_bind_group
    Buffer binding size 0 is less than minimum 4
      note: buffer = `Storage Buffer`

', wgpu/src/backend/direct.rs:3139:5
```

As this was the first example I tried to run, it almost scared me
away, thinking it was a driver issue.

Instead, without arguments the example should use defaults.