Update the URL in an error message

I just followed the instructions in this page and ran into this error
message. The URL in the error message was different.

Change-Id: Iba5be503a8d1bf8050435471a1de7f8323adffbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577881Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834447}

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/win_cross.md
+    https://chromium.googlesource.com/chromium/src/+/HEAD/docs/win_cross.md
 
 then you may need to re-authenticate via: