DOC: Use a valid literal in README

Switch example from using a set of lists (which is invalid because python lists are mutable and hence not hashable) to a set of tuples.
diff --git a/README.rst b/README.rst
index fc63195..1ec01e1 100644
--- a/README.rst
+++ b/README.rst
@@ -279,9 +279,9 @@
 .. code-block:: python
 
     BAZ = {
-        [1, 2, 3, 4],
-        [5, 6, 7, 8],
-        [9, 10, 11, 12]
+        (1, 2, 3, 4),
+        (5, 6, 7, 8),
+        (9, 10, 11, 12),
     }  # yapf: disable
 
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