Broken link in docs

[tdrwenski]

From eSupport (@WardF):

Broken link in https://docs.unidata.ucar.edu/nug/current/netcdf_data_set_components.html

In the section on attributes, the second to last paragraph contains a link to https://www.unidata.ucar.edu/software/netcdf/documentation/4.7.4-pre/attribute_conventions.html ("Appendix A: Attribute Conventions."), when it should be https://docs.unidata.ucar.edu/netcdf-c/current/attribute_conventions.html