Document that `key` must start with a dot in JSON cheats

[PaulRBerg]

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

forge-std/src/Vm.sol

Lines 213 to 239 in e8a047e

	//
	// parseJson
	//
	// ----
	// In case the returned value is a JSON object, it's encoded as a ABI-encoded tuple. As JSON objects
	// don't have the notion of ordered, but tuples do, they JSON object is encoded with it's fields ordered in
	// ALPHABETICAL order. That means that in order to successfully decode the tuple, we need to define a tuple that
	// encodes the fields in the same order, which is alphabetical. In the case of Solidity structs, they are encoded
	// as tuples, with the attributes in the order in which they are defined.
	// For example: json = { 'a': 1, 'b': 0xa4tb......3xs}
	// a: uint256
	// b: address
	// To decode that json, we need to define a struct or a tuple as follows:
	// struct json = { uint256 a; address b; }
	// If we defined a json struct with the opposite order, meaning placing the address b first, it would try to
	// decode the tuple in that order, and thus fail.
	// ----
	// Given a string of JSON, return it as ABI-encoded
	function parseJson(string calldata json, string calldata key) external pure returns (bytes memory abiEncodedData);
	function parseJson(string calldata json) external pure returns (bytes memory abiEncodedData);
	// The following parseJson cheatcodes will do type coercion, for the type that they indicate.
	// For example, parseJsonUint will coerce all values to a uint256. That includes stringified numbers '12'
	// and hex numbers '0xEF'.
	// Type coercion works ONLY for discrete values or arrays. That means that the key must return a value or array, not
	// a JSON object.
	function parseJsonUint(string calldata, string calldata) external returns (uint256);

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

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.