win_cross.md: fix the b/ link that does not work in the generated page

b/<id> link seems not working because the system recognizes it
as a relative URL. Adding http:// should work (for Googlers).

Change-Id: I6348dffa99318ab5eef0a74e20d9fc3c25b23b1c
Reviewed-on: https://chromium-review.googlesource.com/746485Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512786}

docs/win_cross.md

@@ -6,7 +6,7 @@ current restrictions.
 
 What does *not* work:
 
-* goma. Sorry. ([internal bug](b/64390790)) You can use the
+* goma. Sorry. ([internal bug](http://b/64390790)) You can use the
   [jumbo build](jumbo.md) for faster build times.
 * mini_installer ([bug](https://crbug.com/762073))
 * linking on Mac ([bug](https://crbug.com/774209)), should change soon