[gradsusr] Documentation fix: Link for SZIP

[Dave Allured]

Grads team,

On the Supplibs web page, it seems that the link for the SZIP home
page is for an unrelated software package.  The correct link should
be:

http://www.hdfgroup.org/doc_resource/SZIP/

I could not find a more authentic home page.  It would seem that the
HDF Group has adopted the SZIP software distribution.  If anyone knows
of a more authentic website, please let us know.  Thanks for updating
the Supplibs page when you get around to it.

--Dave