Grammatical error describing exit test value capturing

[grynspan]

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

Line 131 in 5ac9500

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 init(from:) before being passed to the exit test body.

Need to clean that up.