Fixed string extensions (backslahses and forward slashes were inverted)

[StephenHodgson]

XRTK - Mixed Reality Toolkit Pull Request

Overview

Back slash and forward slash string extensions were inverted, needed to be fixed.

Submodule Changes

• https://github.com/XRTK/Lumin/pull/112
• https://github.com/XRTK/Oculus/pull/118
• https://github.com/XRTK/SDK/pull/240
• https://github.com/XRTK/WindowsMixedReality/pull/140