Fix errors in OptimizationTips

Author: panzarino

docs/OptimizationTips.rst

@@ -557,6 +557,7 @@ If it makes sense to limit the adoption of protocols to classes then mark
 protocols as class-only protocols to get better runtime performance.
 
 ::
+
   protocol Pingable : class { func ping() -> Int }
 
 .. https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html