Update project description

Problem/Motivation

The project description contains incorrect instructions for getting started with the module:

composer require microsoft/graph
This project does not exist. It should be https://packagist.org/packages/microsoft/microsoft-graph

Go to settings at "/admin/config/graph_mail" and input data from microsoft (aquired by following detailed setup mentioned at step 1)
This URL does not exist. It should be /admin/config/services/graph_mail

Test mail sending, by checking "Send test email" checkbox, inputting "Test email Recipient" and saving the configuration. Test email should be sent to provided email.
Test mail was removed in #3323891: Remove the code that sends a test email, so this line should be removed.

Proposed resolution

Update the project description.