Fix an incorrect file name in tests/data/README.md

The second instance of unsupported_gainmap_version.avif is apparently
a copy-and-paste error. It has an unsupported writer_version (99). It
should be unsupported_gainmap_writer_version_with_extra_bytes.avif.

tests/data/README.md

@@ -551,9 +551,9 @@ by running `./tests/avifgainmaptest --gtest_filter=GainMapTest.CreateGainMapImag
 Contains a gain map with the `minimum_version` field set to 99 in the tmap box.
 `version` and `writer_version` are 0.
 
-### File [unsupported_gainmap_version.avif](unsupported_gainmap_version.avif)
+### File [unsupported_gainmap_writer_version_with_extra_bytes.avif](unsupported_gainmap_writer_version_with_extra_bytes.avif)
 
-![](unsupported_gainmap_version.avif)
+![](unsupported_gainmap_writer_version_with_extra_bytes.avif)
 
 License: [same as libavif](https://github.com/AOMediaCodec/libavif/blob/main/LICENSE)