Correct the idiom for extending lifetime.

README.markdown

@@ -682,16 +682,16 @@ Code (even non-production, tutorial demo code) should not create reference cycle
 
 ### Extending object lifetime
 
-Extend object lifetime using the `[weak self]` and `guard let strongSelf = self else { return }` idiom. `[weak self]` is preferred to `[unowned self]` where it is not immediately obvious that `self` outlives the closure. Explicitly extending lifetime is preferred to optional unwrapping.
+Extend object lifetime using the `[weak self]` and ```guard let `self` = self else { return }``` idiom. `[weak self]` is preferred to `[unowned self]` where it is not immediately obvious that `self` outlives the closure. Explicitly extending lifetime is preferred to optional unwrapping. In Swift 4.2, you should drop the backticks in the guard statement.
 
 **Preferred**
 ```swift
 resource.request().onComplete { [weak self] response in
-  guard let strongSelf = self else {
+  guard let `self` = self else {
     return
   }
-  let model = strongSelf.updateModel(response)
-  strongSelf.updateUI(model)
+  let model = self.updateModel(response)
+  self.updateUI(model)
 }
 ```