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!

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 {