[wwwdocs] Update snapshots.html to refer to sha512sum

[Gerald Pfeifer]

On Thu, 14 Mar 2019, Jonathan Wakely wrote:
> The checksum file uses SHA512 not MD5SUM, so the instructions should be 
> updated accordingly.
> 
> SInce there's only one tarfile these days (not separate ones for
> gcc-base, gcc-g++, gcc-fortran, etc) it doesn't seem necessary to
> filter out the "OK" lines with grep, but I could add --quiet to the
> command if we want that. I added --ignore-missing because I doubt most
> people also download the README and index.html files, so the option
> avoids gettings warnings about those files.

Thank you!

Gerald