Fix WEBGL_compressed_texture_es3.

Remove unnecessary language.
s/COMPRESSED_RGB_ETC2/COMPRESSED_RGB8_ETC2/
Merge pull request #808 from jdashg/fix-es3-ext

Fix WEBGL_compressed_texture_es3.
Fix URL typo.
Merge pull request #809 from jdashg/fix-es3-ext

Fix URL typo.