pgsql: Ensure we use the correct spelling of "ensure"

Ensure we use the correct spelling of "ensure"

We seem to have accidentally used "insure" in a few places.  Correct
that.

Author: Peter Smith
Discussion: https://postgr.es/m/CAHut+Pv0biqrhA3pMhu40aDsj343mTsD75khKnHsLqR8P04f=Q@mail.gmail.com
Backpatch-through: 12, oldest supported version

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/456d697bae081ae77ff222375cf0704316041f88

Modified Files
--------------
src/backend/access/heap/hio.c       | 2 +-
src/backend/utils/misc/guc.c        | 2 +-
src/include/storage/buf_internals.h | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)