x86: Update chromebook_link instructions for binary blob

The MRC image is incorrect, or at least this one now does not seem to
work. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.x86 b/doc/README.x86
index fe31f3d..ef13fb4 100644
--- a/doc/README.x86
+++ b/doc/README.x86
@@ -75,7 +75,7 @@
 
 * ./mainboard/google/link/descriptor.bin
 * ./mainboard/google/link/me.bin
-* ./northbridge/intel/sandybridge/systemagent-ivybridge.bin
+* ./northbridge/intel/sandybridge/systemagent-r6.bin
 
 The 3rd one should be renamed to mrc.bin.
 As for the video ROM, you can get it here [2].