Add new 'update' command to AddressBook1

[mechome]

Add 'update' command to AddressBook.java to update the phone number/email address associated with a name. @Happytreat

[Happytreat]

Nice to see usage of Optional (quite important in AB4) in your enhancement :) In this case however as this particular method is quite short, Optional may not be necessary but in longer code methods Optional will be especially useful.

[mechome]

Okay thanks a lot!

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