Fix typo

[ktakayama]

Fix typo in 0a1927ddfaaca30863ec4ed8ed9350872b775003

[neilpa]

Is it possible to add a test for this? Specifically, is there some string we could use that wouldn't have round-tripped correctly through this before that's easy to detect?

[ktakayama]

I have no idea for test. But, maybe you can create unique key for each method like this.

 public var rex_text: MutableProperty<String?> {
     let key = "\(#line)-\(#function)"
     return associatedProperty(self, key: key, initial: { $0.text }, setter: { $0.text = $1 })
 }