Fix invalid JavaScript in readme example

This switch/typeof example isn't valid JavaScript AFAICT -- JS doesn't have "type switches" like Go does.

NOTE: Unfortunately this example still won't work, because NotFoundError and MethodNotAllowedError are not imported. It doesn't look like kv-asset-handler exports these types currently, so I guess a code change is needed to export them?

See #93 (not fully fixed by this PR)

README.md

@@ -62,14 +62,12 @@ async function handleEvent(event) {
     try {
       return await getAssetFromKV(event)
     } catch (e) {
-        switch (typeof resp) {
-          case NotFoundError: 
-            //..
-          case MethodNotAllowedError:
-            // ...
-          default:
-            return new Response("An unexpected error occurred", { status: 500 })
-        }
+      if (e instanceof NotFoundError) {
+        // ...
+      } else if (e instanceof MethodNotAllowedError) {
+        // ...
+      } else {
+        return new Response("An unexpected error occurred", { status: 500 })
       }
     }
   } else return fetch(event.request)
@@ -190,4 +188,4 @@ A custom handler for mapping requests to a single root: `index.html`. The most c
 import { getAssetFromKV, serveSinglePageApp } from '@cloudflare/kv-asset-handler'
 ...
 let asset = await getAssetFromKV(event, { mapRequestToAsset: serveSinglePageApp })
-```
+```