Change example in pgindent README on "/*-----" comments.

Change example in pgindent README on "/*-----" comments.

Most, but not all, of our "/*----" style comments have an end-guard
line with dashes at the end of the comment. However, pgindent doesn't
care about the end-guards, so they mostly just waste screen
space. Going forward, let's not require end-guards.

Remove a broken end-guard in a comment in walsender.c that led me to
think about this. Remove the end guard from another comment in
walsender.c for consistency, so that we use the same style in all
comments in the file.

However, we have thousands of existing "/*----" comments the repository,
so it's not worth the code churn to change them all.

Discussion: https://www.postgresql.org/message-id/fb083c91-d490-3b65-25f3-05e9118b6b0d%40iki.fi