Problem: nuget packaging refers to imatix gsl

Solution: re-point to the maintained zeromq fork
Signed-off-by: Jim Klimov <EvgenyKlimov@eaton.com>

packaging/nuget/package.gsl

 .#  Generate NuGet nuspec file (for subsequent packing).
 .#
 .#  This is a code generator built using the iMatix GSL code generation
-.#  language. See https://github.com/imatix/gsl for details. This script
+.#  language. See https://github.com/zeromq/gsl for details. This script
 .#  is licensed under MIT/X11.
 .#
 .echo "Generating package.nuspec from template."