Update MRTK_Packages.md to handle spelling error
Update MRTK_Packages.md to handle spelling error

Updates MRTK_Packages.md to replace "handing" with "handling"
killerantz/HoloToolkit-Unity,killerantz/HoloToolkit-Unity,killerantz/HoloToolkit-Unity,DDReaper/MixedRealityToolkit-Unity,killerantz/HoloToolkit-Unity