Update MainWindow.xaml.cs

Changed the download speed indicator to say MB/s instead of Mb/s. The speed indicated is in Megabytes, not Megabits. When tested, before it will show ~5 Mb/s instead of ~40 Mb/s as measured by my router and the Windows 8.1 task manager. Now it will show 5MB/s which is the ~40Mb/s measured. 

tl;dr capitalized to show MegaBytes instead of megabits