jsondocck: Use correct index for error message.

If you misused a count command like `@count $some.selector '"T'"`, you would panic with OOB:

```
thread 'main' panicked at src/tools/jsondocck/src/main.rs:76:92:
index out of bounds: the len is 2 but the index is 2
```

Fixing this typo, we now get.

```
Invalid command: Second argument to @count must be a valid usize (got `"T"`) on line 20
```

As some point I want to rewrite this code to avoid indexing in general, but this is a nice small fix.

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