docs: fix provider stateHandlers example (#1513)

Author: zachlysobey

docs/provider.md

@@ -120,9 +120,9 @@ If you have defined any `state`s in your consumer tests, the `Verifier` can put
 const opts = {
   ...
   stateHandlers: {
-    [null]: () => {
+    "null": () => {
       // This is the "default" state handler, when no state is given
-    }
+    },
     "Has no animals": () => {
       animalRepository.clear()
       return Promise.resolve(`Animals removed from the db`)