Fix typo in path in .gni file.

It's probably not used for anything, since the typo didn't matter.

Change-Id: I5bdea988aeec6dfc142d0bb26e3a438840c45810
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/300242
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>

build/config/win/visual_studio_version.gni

diff --git a/build/config/win/visual_studio_version.gni b/build/config/win/visual_studio_version.gni
index 782d8e8..bdf74e3 100644
--- a/build/config/win/visual_studio_version.gni
+++ b/build/config/win/visual_studio_version.gni
@@ -19,7 +19,7 @@
   # Full path to the Windows SDK, not including a backslash at the end.
   # This value is the default location, override if you have a different
   # installation location.
-  windows_sdk_path = "C:\Program Files (x86)\Windows Kits\8.1"
+  windows_sdk_path = "C:\\Program Files (x86)\\Windows Kits\\8.1"
 }
 
 if (visual_studio_path == "") {