pgsql: docs: land height is "elevation", not "altitude"

docs: land height is "elevation", not "altitude"

See https://mapscaping.com/blogs/geo-candy/what-is-the-difference-between-elevation-relief-and-altitude
No patching of regression tests.

Reported-by: taf1@cornell.edu

Discussion: /messages/by-id/158506544539.679.2278386310645558048@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c5abde5f350d06cd62f0f1244dfcaebc5eaf1fc5

Modified Files
--------------
doc/src/sgml/advanced.sgml | 42 ++++++++++++++--------------
doc/src/sgml/ddl.sgml | 70 +++++++++++++++++++++++-----------------------
2 files changed, 56 insertions(+), 56 deletions(-)