fix: typo in custom-session-token docs (#1787)

docs/backend-requests/making/custom-session-token.mdx

@@ -40,11 +40,11 @@ This guide will show you how to customize a session token to include additional
     export default async function Page() {
       const { sessionClaims } = await auth()
 
-      const firstName = sessionClaims?.fullName
+      const fullName = sessionClaims?.fullName
 
       const primaryEmail = sessionClaims?.primaryEmail
 
-      return NextResponse.json({ firstName, primaryEmail })
+      return NextResponse.json({ fullName, primaryEmail })
     }
     ```
 
@@ -55,11 +55,11 @@ This guide will show you how to customize a session token to include additional
     export default async function handler(req: NextApiRequest, res: NextApiResponse) {
       const { sessionClaims } = getAuth(req)
 
-      const firstName = sessionClaims.fullName
+      const fullName = sessionClaims.fullName
 
       const primaryEmail = sessionClaims.primaryEmail
 
-      return res.status(200).json({ firstName, primaryEmail })
+      return res.status(200).json({ fullName, primaryEmail })
     }
     ```
   </CodeBlockTabs>
@@ -73,14 +73,14 @@ This guide will show you how to customize a session token to include additional
   1. Create the `CustomJwtSessionClaims` interface and declare it globally.
   1. Add the custom claims to the `CustomJwtSessionClaims` interface.
 
-  The following example demonstrates how to add the `firstName` and `primaryEmail` claims to the `CustomJwtSessionClaims` interface.
+  The following example demonstrates how to add the `fullName` and `primaryEmail` claims to the `CustomJwtSessionClaims` interface.
 
   ```tsx {{ filename: 'types/globals.d.ts' }}
   export {}
 
   declare global {
     interface CustomJwtSessionClaims {
-      firstName?: string
+      fullName?: string
       primaryEmail?: string
     }
   }