pgsql: Fix two typos in xlogstats.h and xlogstats.c

Fix two typos in xlogstats.h and xlogstats.c

Issue found while browsing this area of the code, introduced and
copy-pasted around by 2258e76f90bf.

Backpatch-through: 15

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ed047ce0a881e628a17e0a3fe6a19c0faa155563

Modified Files
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src/backend/access/transam/xlogstats.c | 2 +-
src/include/access/xlogstats.h | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)