Fix a typo in exit tests documentation. (#1414)

Add a missing word in the exit tests documentation.

Resolves #1372.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Testing.docc/exit-testing.md

@@ -128,7 +128,7 @@ value using the `as` operator:
 Every value you capture in an exit test must conform to [`Sendable`](https://developer.apple.com/documentation/swift/sendable)
 and [`Codable`](https://developer.apple.com/documentation/swift/codable). Each
 value is encoded by the parent process using [`encode(to:)`](https://developer.apple.com/documentation/swift/encodable/encode(to:))
-and is decoded by the child process [`init(from:)`](https://developer.apple.com/documentation/swift/decodable/init(from:))
+and is decoded by the child process using [`init(from:)`](https://developer.apple.com/documentation/swift/decodable/init(from:))
 before being passed to the exit test body.
 
 If a captured value's type does not conform to both `Sendable` and `Codable`, or