pgsql: doc: Minor wordsmithing to COPY docs

doc: Minor wordsmithing to COPY docs

Perform some minor wordsmithing on two sentences in the COPY documentation
to make them clearer.

While there, also ensure to wrap a few occurrences of CSV in <literal>
which were missing this.

Reported-by: Eric Mutta <eric.mutta@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/166104548566.654.11680826843612576896@wrigleys.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e2fe0aedd00eae89e6d00bca5c598d1ac31b266a

Modified Files
--------------
doc/src/sgml/ref/copy.sgml | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)