pgsql: doc: Add missing tags in pg_buffercache page

doc: Add missing tags in pg_buffercache page

Issue noticed while looking at this area of the documentation, for a
different patch.  This is a matter of style, so no backpatch is done.

Discussion: https://postgr.es/m/CAN55FZ0h_YoSqqutxV6DES1RW8ig6wcA8CR9rJk358YRMxZFmw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c75bf57a90e05bf35afa00e66986b9210833bdaf

Modified Files
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doc/src/sgml/pgbuffercache.sgml | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)