pgsql: Prefer spelling "cacheable" over "cachable".

Prefer spelling "cacheable" over "cachable".

Previously we had both in code and comments.  Keep the more common and
accepted variant.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/5EBF1771-0566-4D08-9F9B-CDCDEF4BDC98@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2421ade663e96e7245e6bbf960b0f6861e39e455

Modified Files
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src/backend/access/heap/heapam.c   | 4 ++--
src/backend/utils/cache/catcache.c | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)