PEP 654: Tidy up leaf_generator example (#1854)

Author: iritkatriel

pep-0654.rst

@@ -414,14 +414,14 @@ We can then process the full tracebacks of the leaf exceptions:
     ...     eg = e
     ...
     >>> for (i, (exc, tbs)) in enumerate(leaf_generator(eg)):
-    ...     print(f"\n>>> Exception #{i+1}:")
+    ...     print(f"\n=== Exception #{i+1}:")
     ...     traceback.print_exception(exc)
     ...     print(f"The complete traceback for Exception #{i+1}:")
     ...     for tb in tbs:
     ...         traceback.print_tb(tb)
     ...
 
-    >>> Exception #1:
+    === Exception #1:
     Traceback (most recent call last):
       File "<stdin>", line 3, in g
     ValueError: 1
@@ -430,7 +430,7 @@ We can then process the full tracebacks of the leaf exceptions:
       File "<stdin>", line 2, in f
       File "<stdin>", line 3, in g
 
-    >>> Exception #2:
+    === Exception #2:
     Traceback (most recent call last):
       File "<stdin>", line 3, in g
     ValueError: 2