Fix help message for --no-namespace-packages. (#17472)

Fixes #17466

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Change help message of '--no-namespace-packages' option

Co-authored-by: Raphael Krupinski <10319569-mattesilver@users.noreply.gitlab.com>

mypy/main.py

@@ -565,7 +565,7 @@ def add_invertible_flag(
         "--no-namespace-packages",
         dest="namespace_packages",
         default=True,
-        help="Support namespace packages (PEP 420, __init__.py-less)",
+        help="Disable support for namespace packages (PEP 420, __init__.py-less)",
         group=imports_group,
     )
     imports_group.add_argument(