Rename megasync.svg to mega.svg to properly display MEGAsync icon

The icon wasn't displaying before because it should be named "mega", not "megasync" (the MEGAsync client uses the icon name "mega"):

A simple file rename fixes that and it displays correctly now. I tested it on my local machine and it works.

Before filename change (just displaying the default MEGAsync icon):

After filename change (displaying the proper Tela icon):