Fix typo in README

Fix typo in //media/test/README: s/@/2/. No new code.

TBR=kcwu@chromium.org

Change-Id: Iee9e1d086498d1fce340576f0d32db42920ac103
Reviewed-on: https://chromium-review.googlesource.com/804055Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521126}

media/test/data/README

@@ -200,7 +200,7 @@ test-25fps.vp9_2:
   configured with the option --enable-vp9-highbitdepth).
 
 test-25fps.vp9_2.md5:
-  MD5 of RGB thumbnail rendered version of test-25fps.vp9_@. Written out by
+  MD5 of RGB thumbnail rendered version of test-25fps.vp9_2. Written out by
   video_decode_accelerator_unittest.
 
 // VDA test files: bear