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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3a36543d7d08eef6496fe3511f4cf04cba14f572

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