Fix typo: yaml -> json (#17854)

lib/spack/spack/cmd/spec.py

@@ -34,7 +34,7 @@ def setup_parser(subparser):
         const='yaml', help='print concrete spec as YAML')
     subparser.add_argument(
         '-j', '--json', action='store_const', dest='format', default=None,
-        const='json', help='print concrete spec as YAML')
+        const='json', help='print concrete spec as JSON')
     subparser.add_argument(
         '-c', '--cover', action='store',
         default='nodes', choices=['nodes', 'edges', 'paths'],