doc (fundamentals.rst): Fix grammar in timezone-aware DAG description (#49855)

Add missing "is" verb in sentence about creating timezone-aware DAGs

(cherry picked from commit 7b6a6f7)

Author: mohamedhayballa22

airflow-core/docs/tutorial/fundamentals.rst

@@ -210,7 +210,7 @@ times.
 Working with Time Zones
 -----------------------
 
-Creating a time zone aware DAG straightforward. Just ensure you use time zone aware dates
+Creating a time zone aware DAG is straightforward. Just ensure you use time zone aware dates
 with ``pendulum``. Avoid using the standard library
 `timezone <https://docs.python.org/3/library/datetime.html#timezone-objects>`_ as they have known limitations.