Update rbac.md - code match (redwoodjs#10405)

code in repo was different from code in tutorial and it was missing the
if statement. i added it in to reduce confusion for a new person who
might get confused by the difference.

tutorial code: 
![Screen Shot 2024-04-03 at 12 06 55
PM](https://github.com/redwoodjs/redwood/assets/2617706/4847d98a-5ed4-4e51-9cb6-ef6e91cac432)


repo code: 
![Screen Shot 2024-04-03 at 12 07 34
PM](https://github.com/redwoodjs/redwood/assets/2617706/210655e5-4ac5-434c-a08f-1a158566d3d9)

docs/docs/tutorial/chapter7/rbac.md

@@ -72,6 +72,10 @@ Before we do that, we'll need to make sure that the web side has access to the r
 
 ```javascript title="api/src/lib/auth.js"
 export const getCurrentUser = async (session) => {
+  if (!session || typeof session.id !== 'number') {
+    throw new Error('Invalid session')
+  }
+
   return await db.user.findUnique({
     where: { id: session.id },
     // highlight-next-line