DeveloperGuide: Fix formatting errors in the use case
DeveloperGuide: Fix formatting errors in the use case

There are formatting errors in the use cases. For example, some use
cases are not properly indented.

Let's fix them.

CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,se-edu/addressbook-level3,CS2103R-Eugene-Peh/addressbook-level4