Update Swift pre-decrement syntax to match v3.0 (#2395)

swift.html.markdown

@@ -94,7 +94,7 @@ var unwrappedString: String! = "Value is expected."
 // same as above, but ! is a postfix operator (more syntax candy)
 var unwrappedString2: ImplicitlyUnwrappedOptional<String> = "Value is expected."
 
-// If let structure - 
+// If let structure -
 // If let is a special structure in Swift that allows you to check if an Optional rhs holds a value, and in case it does - unwraps and assigns it to the lhs.
 if let someOptionalStringConstant = someOptionalString {
     // has `Some` value, non-nil
@@ -443,7 +443,7 @@ internal class Rect: Shape {
 
     func shrink() {
         if sideLength > 0 {
-            --sideLength
+            sideLength -= 1
         }
     }