pythongh-106670: Fix a typo in doc of pdb's exceptions command (python#132277)

Author: gaogaotiantian

Doc/library/pdb.rst

@@ -769,7 +769,7 @@ can be overridden by the local file.
    When using ``pdb.pm()``  or ``Pdb.post_mortem(...)`` with a chained exception
    instead of a traceback, it allows the user to move between the
    chained exceptions using ``exceptions`` command to list exceptions, and
-   ``exception <number>`` to switch to that exception.
+   ``exceptions <number>`` to switch to that exception.
 
 
    Example::