Documentation: exception-tables.rst: Fix the wrong steps referenced

Documentation: exception-tables.rst: Fix the wrong steps referenced

When it was in text format, it correctly hardcoded steps 8a to 8c.
However, after it was converted to RST, the sequence numbers were
auto-generated during rendering and became incorrect after some
steps were inserted.

Change it to refer to steps a to c in a relative way.

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
[jc: Indented the line to make the relative reference more clear]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240614010028.48262-1-lizhijian@fujitsu.com