pgsql: doc: Update header file mention for CompareType

doc: Update header file mention for CompareType

Commit 119fc30 moved CompareType to cmptype.h but the mention in
the docs still refered to primnodes.h

Author: Daisuke Higuchi <higuchi.daisuke11@gmail.com>
Reviewed-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAEVT6c8guXe5P=L_Un5NUUzCgEgbHnNcP+Y3TV2WbQh-xjiwqA@mail.gmail.com
Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b30089fde1e9e945f96db9366f5ff5eb4abd3774

Modified Files
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doc/src/sgml/gist.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)