Update extensibility.md

two dimension -> two dimensions

src/pages/codata/extensibility.md

@@ -2,7 +2,7 @@
 
 We have seen that codata can represent types with an infinite number of elements, such as `Stream`. This is one expressive difference from data, which must always be finite. We'll now look at another, which is the type of extensibility we get from data and from codata. Together these gives use guidelines to choose between the two.
 
-Firstly, let's define extensibility. It means the ability to add new features without modifying existing code. (If we allow modification of existing code then any extension becomes trivial.) In particular there are two dimension along which we can extend code: adding new functions or adding new elements. We will see that data and codata have orthogonal extensibility: it's easy to add new functions to data but adding new elements is impossible without modifying existing code, while adding new elements to codata is straight-forward but adding new functions is not.
+Firstly, let's define extensibility. It means the ability to add new features without modifying existing code. (If we allow modification of existing code then any extension becomes trivial.) In particular there are two dimensions along which we can extend code: adding new functions or adding new elements. We will see that data and codata have orthogonal extensibility: it's easy to add new functions to data but adding new elements is impossible without modifying existing code, while adding new elements to codata is straight-forward but adding new functions is not.
 
 Let's start with a concrete example of both data and codata. For data we'll use the familiar `List` type.