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 <[e-mail]>
[jc: Indented the line to make the relative reference more clear]
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/arch/x86/exception-tables.rst

···
    c) execution continues at local label 2 (address of the
       instruction immediately after the faulting user access).
 
-The steps 8a to 8c in a certain way emulate the faulting instruction.
+   The steps a to c above in a certain way emulate the faulting instruction.
 
 That's it, mostly. If you look at our example, you might ask why
 we set EAX to -EFAULT in the exception handler code. Well, the