Fixed docs to refer to HSTS includeSubdomains as a directive.

The spec refers to it as a 'directive' rather than a 'tag':
https://tools.ietf.org/html/rfc6797#section-6.1.2

docs/ref/middleware.txt

@@ -255,8 +255,8 @@ so that infrequent visitors will be protected (31536000 seconds, i.e. 1 year,
 is common).
 
 Additionally, if you set the :setting:`SECURE_HSTS_INCLUDE_SUBDOMAINS` setting
-to ``True``, ``SecurityMiddleware`` will add the ``includeSubDomains`` tag to
-the ``Strict-Transport-Security`` header. This is recommended (assuming all
+to ``True``, ``SecurityMiddleware`` will add the ``includeSubDomains`` directive
+to the ``Strict-Transport-Security`` header. This is recommended (assuming all
 subdomains are served exclusively using HTTPS), otherwise your site may still
 be vulnerable via an insecure connection to a subdomain.
 

docs/ref/settings.txt

@@ -2053,7 +2053,7 @@ already have it.
 Default: ``False``
 
 If ``True``, the :class:`~django.middleware.security.SecurityMiddleware` adds
-the ``includeSubDomains`` tag to the :ref:`http-strict-transport-security`
+the ``includeSubDomains`` directive to the :ref:`http-strict-transport-security`
 header. It has no effect unless :setting:`SECURE_HSTS_SECONDS` is set to a
 non-zero value.
 

tests/middleware/test_security.py

@@ -83,7 +83,7 @@ class SecurityMiddlewareTest(SimpleTestCase):
         """
         With HSTS_SECONDS non-zero and HSTS_INCLUDE_SUBDOMAINS
         True, the middleware adds a "strict-transport-security" header with the
-        "includeSubDomains" tag to the response.
+        "includeSubDomains" directive to the response.
         """
         response = self.process_response(secure=True)
         self.assertEqual(response["strict-transport-security"], "max-age=600; includeSubDomains")
@@ -94,7 +94,7 @@ class SecurityMiddlewareTest(SimpleTestCase):
         """
         With HSTS_SECONDS non-zero and HSTS_INCLUDE_SUBDOMAINS
         False, the middleware adds a "strict-transport-security" header without
-        the "includeSubDomains" tag to the response.
+        the "includeSubDomains" directive to the response.
         """
         response = self.process_response(secure=True)
         self.assertEqual(response["strict-transport-security"], "max-age=600")