fix(cmd spec -y): add yaml doc separators (---)

lib/spack/spack/cmd/spec.py

@@ -100,9 +100,14 @@ def spec(parser, args):
 
     # With --yaml, --json, or --format, just print the raw specs to output
     if args.format:
+        first_doc = True
         for spec in concrete_specs:
             if args.format == "yaml":
                 # use write because to_yaml already has a newline.
+                if first_doc:
+                    first_doc = False
+                else:
+                    sys.stdout.write("---\n")
                 sys.stdout.write(spec.to_yaml(hash=ht.dag_hash))
             elif args.format == "json":
                 print(spec.to_json(hash=ht.dag_hash))