Grammar: change "object's" to "an object's" and add commas

Also change "parameterized" to "parameterised" to follow the
British spelling convention used elsewhere.

Author: neuroevolutus

_tour/higher-order-functions.md

@@ -16,7 +16,7 @@ The terminology can get a bit confusing at this point, and we use the phrase
 "higher order function" for both methods and functions that take functions as parameters
 or that return a function.
 
-In a pure Object Oriented world a good practice is to avoid exposing methods parameterized with functions that might leak object's internal state. Leaking internal state might break the invariants of the object itself thus violating encapsulation.
+In a pure Object Oriented world, a good practice is to avoid exposing methods parameterised with functions that might leak an object's internal state. Leaking internal state might break the invariants of the object itself, thus violating encapsulation.
 
 One of the most common examples is the higher-order
 function `map` which is available for collections in Scala.