Replace github.com/llnl/spack with github.com/spack/spack (#6142)

We moved to a new GitHub org! Now make the code and docs reflect that.

templates/modules/modulefile.lua

@@ -1,5 +1,5 @@
 -- -*- lua -*-
--- Module file created by spack (https://github.com/LLNL/spack) on {{ timestamp }}
+-- Module file created by spack (https://github.com/spack/spack) on {{ timestamp }}
 --
 -- {{ spec.short_spec }}
 --
@@ -88,4 +88,4 @@ unsetenv("{{ cmd.name }}")
 
 {% block footer %}
 {# In case the module needs to be extended with custom LUA code #}
-{% endblock %}
+{% endblock %}