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

[Ryan Lovelett]

I am proposing that https://github.com/apple/swift/pull/4177 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!