pgsql: Fix comments related to table AMs

Fix comments related to table AMs

Incorrect function names were referenced. As this fixes some portions
of tableam.h, that is mentioned in the docs as something to look at when
implementing a table AM, backpatch down to 12 where this has been
introduced.

Author: Hironobu Suzuki
Discussion: /messages/by-id/8fe6d672-28dd-3f1d-7aed-ac2f6d599d3f@interdb.jp
Backpatch-through: 12

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9168793d7275b4b318c153d607fba55d14098c19

Modified Files
--------------
src/backend/access/heap/heapam.c | 6 +++---
src/include/access/tableam.h | 21 +++++++++++----------
2 files changed, 14 insertions(+), 13 deletions(-)