namespaces.7: ffix

Put a paragraph break above the table to ensure separation from the
preceding paragraph with the man(7) macros from groff >= 1.23 and
forthcoming mandoc(1) release.

Draw a horizontal rule under the column headings.

Link: <https://cvsweb.bsd.lv/mandoc/man_term.c?rev=1.241&content-type=text/x-cvsweb-markup>
Signed-off-by: "G. Branden Robinson" <g.branden.robinson@gmail.com>
Message-ID: <20240901032453.3dmhjz7urk2saizq@illithid>
Signed-off-by: Alejandro Colomar <alx@kernel.org>