Correct the sweep order example in the docstring (#7104)

Author: ikd-sci

cirq-core/cirq/study/sweeps.py

@@ -216,7 +216,8 @@ class Product(Sweep):
     If one sweep assigns 'a' to the values 0, 1, 2, and the second sweep
     assigns 'b' to the values 2, 3, then the product is a sweep that
     assigns the tuple ('a','b') to all possible combinations of these
-    assignments: (0, 2), (1, 2), (2, 2), (0, 3), (1, 3), (2, 3).
+    assignments: (0, 2), (0, 3), (1, 2), (1, 3), (2, 2), (2, 3).
+    That is, the leftmost sweep is the outer loop in a product of sweeps.
     """
 
     def __init__(self, *factors: Sweep) -> None: