pgsql: Fix comment in pg_get_shmem_allocations_numa()

Fix comment in pg_get_shmem_allocations_numa()

The comment fixed in this commit described the function as dealing with
database blocks, but in reality it processes shared memory allocations.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/aH4DDhdiG9Gi0rG7@ip-10-97-1-34.eu-west-3.compute.internal
Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b2abcfa33a7c2184d96fe62029195773bb8a4032

Modified Files
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src/backend/storage/ipc/shmem.c | 15 ++++++---------
1 file changed, 6 insertions(+), 9 deletions(-)