Corrects code that would fail due to type mismatch

I know this probably seems nit-picky since this isn't a language reference, but it seems numberOfWheels should be converted to Float when doing the math here to get tire pressure. Otherwise, I'm loving this style guide!

Author: kevindeleon

README.md

@@ -190,7 +190,7 @@ class Vehicle {
  }
 
  func maximumTotalTirePressure(pressurePerWheel: Float) -> Float {
- return pressurePerWheel * numberOfWheels
+ return pressurePerWheel * Float(numberOfWheels)
  }
 }
 
@@ -215,7 +215,7 @@ protocol Vehicle {
 }
 
 func maximumTotalTirePressure(vehicle: Vehicle, pressurePerWheel: Float) -> Float {
- return pressurePerWheel * vehicle.numberOfWheels
+ return pressurePerWheel * Float(vehicle.numberOfWheels)
 }
 
 struct Bicycle: Vehicle {