Fix typo

Closes GH-21.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

readme.md

@@ -477,7 +477,7 @@ import type {Data, Literal} from 'mdast'
 
 interface Toml extends Literal {
   type: 'toml'
-  data?: TomlData
+  data?: Data
 }
 
 declare module 'mdast' {