Fix pg_waldump documentation about block option

Hi,

When I read the documentation of pg_waldump, I found the description about
-B option is wrong.

       -B block
       --block=block
           Only display records that modify the given block. The relation must also be provided with --relation or -l.

Before 52b5568, the -l option is short for --relation, however, it has been
changed to -R, and we forgot to update the documentation.

Here is a patch for it.

-- 
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.