[swift-3.0-branch] Fix typo in `sourcekitd-test` error handler response

I am proposing that [SourceKit] Fix typo in `sourcekitd-test` error handler response by RLovelett · Pull Request #4177 · apple/swift · GitHub be reviewed
for acceptance in Swift 3.0.

(Not really sure what the format of this e-mail should be, subject or
body. Sorry if this is wrong.)

Thank you!