Doc fix and adjustment for MERGE command

Hello hackers!

Reading docs for the MERGE statement I've found a little error: a
semicolon in middle of a statement and absence of a semicolon in the end
of it.

Key words in subqueries are written in uppercase everywhere in the docs
but not in an example for MERGE. I think it should be adjusted too.

Also aliases, table and column names are written in lowercase
(snake_case) almost all over the docs. I did not dare to fix examples in
the same patch (may be that style was intentional), but guess that style
of the first two examples should not differ from the third one and from
other examples in docs.

Discussions about MERGE was:
https://postgr.es/m/20220801145257.GA15006@telsasoft.com
https://postgr.es/m/20220714162618.GH18011@telsasoft.com

but I did not find there (via quick search) anything about case styling.

Thank all a lot in advance!

--
Best regards,
Vitaly Burovoy