App allows users to add in emails without a period symbol in the domain

[Runus01]

Description: The AddressBook app allows users to add emails that do not have the period symbol, potentially recording erroneous emails.

Steps to Reproduce:

1. Open the AddressBook app.

2. Enter a command to add a new contact with an email without a '.' in the domain.

   Example command:

   add r/volunteer n/John Doe p/98765432 e/johnd@examplecom a/311, Clementi Ave 2, #02-25 t/friends t/owesMoney h/30
   
   

Expected Result:
The app rejects the contact information with an error message.

Actual Result:
The app accepts the contact information.