[2.1.x] Updated Jinja2 static tag example to use django.templatetags.

As of cf546e11ac, this is the
canonical way to invoke the static tag.
Backport of 084573c715 from master.

docs/topics/templates.txt

@@ -451,7 +451,7 @@ environment.
 
 For example, you can create ``myproject/jinja2.py`` with this content::
 
-    from django.contrib.staticfiles.storage import staticfiles_storage
+    from django.templatetags.static import static
     from django.urls import reverse
 
     from jinja2 import Environment
@@ -460,7 +460,7 @@ For example, you can create ``myproject/jinja2.py`` with this content::
     def environment(**options):
         env = Environment(**options)
         env.globals.update({
-            'static': staticfiles_storage.url,
+            'static': static,
             'url': reverse,
         })
         return env