Fixing syntax error in todos.md (#57)

* Fixing syntax error & TypeScript warning in todos.md

This fixes two small code issues that I came across was I was following the tutorial:

First, a syntax error in `todoActions$`.

Secondly, a TypeScript warning in the definition of `useTodos`:
```
Type 'IterableIterator<Todo>' can only be iterated through when using the '--downlevelIteration' flag or with a '--target' of 'es2015' or higher.
```

Replacing `[...todosMap.values()]` with `Array.from(todosMap.values())` makes that warning go away without customising TypeScript from its default value.

* Revert TypeScript warning

docs/tutorial/todos.md

@@ -51,10 +51,10 @@ Which is basically the same as doing this (but a lot shorter, of course :smile:)
 
 ```tsx
 const todoActions$ = merge(
-  newTodo$.pipe(map(text, id) => ({
-    type: "add" as const
+  newTodo$.pipe(map((text, id) => ({
+    type: "add" as const,
     payload: { id, text },
-  })),
+  }))),
   editTodo$.pipe(map(payload => ({
     type: "edit" as const,
     payload,