docs(StdJson): document that key parameter must start with a dot

The current documentation for the JSON cheats does not explain that the `key` parameter must be provided with a leading dot:

https://github.com/foundry-rs/forge-std/blob/e8a047e3f40f13fa37af6fe14e6e06283d9a060e/src/Vm.sol#L213-L239

I was trying to access the key by its simple name, like so:

```solidity
string memory json = '{"bar":"Hello World"}';
string memory bar = vm.parseJsonString(json, "bar");
```

And that didn't work until I added the leading dot, i.e. `.bar`.

Side note: the parameters for the `parseJsonSOMETHING` cheats have been left unnamed.