win_cross.md: Fix typo

The script is called run-swarmed.py, not run_swarmed.py.

TBR=thakis

Bug: 495204
Change-Id: I43953019fe4884fac9c7491e87845ef7476091fa
Reviewed-on: https://chromium-review.googlesource.com/980053Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545756}

docs/win_cross.md

@@ -62,9 +62,9 @@ to the Windows box and run it to install the chrome you just built.
 
 You can run the Windows binaries you built on swarming, like so:
 
-    tools/run_swarmed.py -C out/gnwin -t base_unittests [ --gtest_filter=... ]
+    tools/run-swarmed.py -C out/gnwin -t base_unittests [ --gtest_filter=... ]
 
-See the contents of run_swarmed.py for how to do this manually.
+See the contents of run-swarmed.py for how to do this manually.
 
 There's a bot doing 64-bit release cross builds at
 https://ci.chromium.org/buildbot/chromium.clang/linux-win_cross-rel/