docs: fix spack command for unit-test pytest help (#20415)

lib/spack/spack/cmd/unit_test.py

@@ -146,7 +146,7 @@ def add_back_pytest_args(args, unknown_args):
 def unit_test(parser, args, unknown_args):
     if args.pytest_help:
         # make the pytest.main help output more accurate
-        sys.argv[0] = 'spack test'
+        sys.argv[0] = 'spack unit-test'
         return pytest.main(['-h'])
 
     # add back any parsed pytest args we need to pass to pytest