Update README.md to mark a codeblock as being swift

README.md

@@ -454,7 +454,7 @@ let string = header.render()
 
 Just like nodes, components can also be rendered on their own:
 
-```
+```swift
 let header = Header {
     H1("Title")
     Span("Description")
@@ -588,4 +588,4 @@ This project does [not come with GitHub Issues-based support](CONTRIBUTING.md#bu
 
 If you wish to make a change, [open a Pull Request](https://github.com/JohnSundell/Plot/pull/new) — even if it just contains a draft of the changes you’re planning, or a test that reproduces an issue — and we can discuss it further from there. See [Plot’s contribution guide](CONTRIBUTING.md) for more information about how to contribute to this project.
 
-Hope you’ll enjoy using Plot!
+Hope you’ll enjoy using Plot!