Ensure proper full stops in all documentation

[Zhiyuan-Amos]

For example, in the User Guide, under Quick Start (Section 1) point 5:

5. Some example commands you can try:
   * **`list`** : lists all contacts
   * **`add`**` John Doe p/98765432 e/[email protected] a/John street, block 123, #01-01` :
     adds a contact named `John Doe` to the Address Book.
   * **`delete`**` 3` : deletes the 3rd contact shown in the current list
   * **`exit`** : exits the app

For the example on add command, there is a full stop at the end of the sentence, whereas there aren't full stops at the end of the explanations for the other commands.