docs: fix hcldec guide example and DecodeBody reference

[gajjuu]

Summary

Fixes two documentation issues in guide/go_decoding_hcldec.rst:

• Replaces undefined variable block.Type with sc.Type in the partial decoding example.
• Replaces nonexistent gohcl.DecodeBlock reference with gohcl.DecodeBody.

Fixes #800

[hashicorp-cla-app]

All committers have signed the CLA.

[crw]

Thanks, I will raise this in triage.