docs: Update readme to include strict param example for exact payload matching

[johnemersonatdoshii]

As stated in the README, including the third param and setting it to true to enable strict payload matching, but the third parameter was not provided. PR includes the third parameter 👍🏽

aws-sdk-client-mock/README.md

Lines 166 to 177 in 3981b97

	Not all payload parameters must be defined to match
	(you can force strict matching by passing third param `strict: true`):
	```typescript
	snsMock
	.on(PublishCommand, {
	Message: 'My message',
	})
	.resolves({
	MessageId: '12345678-4444-5555-6666-111122223333',
	});
	```