Add missing word (kodecocodes#243)

"rather than an instance" -> "rather than on an instance"

Author: jawwad

README.markdown

@@ -515,7 +515,7 @@ Constants are defined using the `let` keyword, and variables with the `var` keyw
 
 **Tip:** A good technique is to define everything using `let` and only change it to `var` if the compiler complains!
 
-You can define constants on a type rather than an instance of that type using type properties. To declare a type property as a constant simply use `static let`. Type properties declared in this way are generally preferred over global constants because they are easier to distinguish from instance properties. Example:
+You can define constants on a type rather than on an instance of that type using type properties. To declare a type property as a constant simply use `static let`. Type properties declared in this way are generally preferred over global constants because they are easier to distinguish from instance properties. Example:
 
 **Preferred:**
 ```swift