UserGuide.md: add missing numbers to ToC links
UserGuide.md: add missing numbers to ToC links

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