fix(permit2): Update example witnessTypeString contents

[gibson042]

Description

Remove some incorrect stray text in the witnessTypeString example at https://docs.uniswap.org/contracts/permit2/reference/signature-transfer#batch-permitwitnesstransferfrom , and fix some nearby references to EIP-712.

Type(s) of changes

• Bug fix
• New feature
• Update to an existing feature

Motivation for PR

Fix incorrect example.

How Has This Been Tested?

n/a

Follow-up PR

Given the emphasis on proper ordering of struct types with the injection of a not-obviously-relevant TokenPermissions, it would be nice for the example entry-point type to actually reference some other structs. For example:

 ```solidity
 struct ExampleTrade {
 	address exampleTokenAddress;
 	uint256 exampleMinimumAmountOut;
+	TradeCondition condition;
 }
+struct TradeCondition {
+	string language;
+	string expression;
+}
 ```
 Following EIP-712, the typehash for the data would be defined by:
 ```solidity
-bytes32 _EXAMPLE_TRADE_TYPEHASH = keccak256('ExampleTrade(address exampleTokenAddress,uint256 exampleMinimumAmountOut)');
+bytes32 _EXAMPLE_TRADE_TYPEHASH = keccak256('ExampleTrade(address exampleTokenAddress,uint256 exampleMinimumAmountOut,TradeCondition condition)TradeCondition(string language,string expression)');
 ```
 The `witness` that should be passed along with the permit message should be:
 ```solidity
  bytes32 witness = keccak256(
-            abi.encode(_EXAMPLE_TRADE_TYPEHASH, exampleTrade.exampleTokenAddress, exampleTrade.exampleMinimumAmountOut));
+            abi.encode(_EXAMPLE_TRADE_TYPEHASH, exampleTrade.exampleTokenAddress, exampleTrade.exampleMinimumAmountOut, (exampleTrade.condition.language, exampleTrade.condition.expression)));
 ```
 And the `witnessTypeString` to be passed in should be:
 
 ```solidity
-string constant witnessTypeString = "ExampleTrade(address exampleTokenAddress,uint256 exampleMinimumAmountOut)TokenPermissions(address token,uint256 amount)"
+string constant witnessTypeString = "ExampleTrade(address exampleTokenAddress,uint256 exampleMinimumAmountOut,TradeCondition condition)TokenPermissions(address token,uint256 amount)TradeCondition(string language,string expression)"

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