pgsql: Doc: Move pg_stat_replication_slots view to "Collected Statistic

Doc: Move pg_stat_replication_slots view to "Collected Statistics Views" section.

Commit 9868167500 added pg_stat_replication_slots view to monitor
ReorderBuffer stats but mistakenly added it under
"Dynamic Statistics Views" section in the docs whereas it belongs to
"Collected Statistics Views" section.

Author: Amit Kapila
Reviewed-by: Masahiko Sawada
Backpatch-through: 14, where it was introduced
Discussion: https://postgr.es/m/CAA4eK1Kb5ur=OC-G4cAsqPOjoVe+S8LNw1WmUY8Owasjk8o5WQ@mail.gmail.com

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/8de4a31720c2c2ce55f73f4af4b3cdc6cdcca225

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)