win_cross.md: Fix to point to correct error message.

The error message points to win_cross.md, as can be seen in the script
https://source.chromium.org/chromium/chromium/tools/depot_tools/+/master:win_toolchain/get_toolchain_if_necessary.py;l=531

Bug: none
Change-Id: I562653b401ec039db4f8a83b4bc38e11f8d4d5db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577459Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Richard Stotz <rstz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834203}

docs/win_cross.md

@@ -41,7 +41,7 @@ If you're at Google, this will automatically download the Windows SDK for you.
 If this fails with an error:
 
     Please follow the instructions at
-    https://chromium.googlesource.com/chromium/src/+/master/docs/windows_build_instructions.md
+    https://chromium.googlesource.com/chromium/src/+/master/docs/win_cross.md
 
 then you may need to re-authenticate via: