Update testvector download command.

Migrating opus-codec.org to a static site generator inserted
a redirect into the download path, which the actual testvectors
at https://opus-codec.org/static/testvectors/opus_testvectors.tar.gz.

Add the -L option to the suggested curl invocation so the redirect
is followed to download the actual resource. Without this one
gets the html redirection fallback page.