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

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

This is a follow-on patch to 92c12e46d5. In that patch, we renamed
"altitude" to "elevation" in the docs, based on these details:

https://mapscaping.com/blogs/geo-candy/what-is-the-difference-between-elevation-relief-and-altitude

This renames the tutorial SQL files to match the documentation.

Reported-by: max1(at)inbox(dot)ru

Discussion: https://postgr.es/m/161512392887.1046.3137472627109459518@wrigleys.postgresql.org

Backpatch-through: 9.6

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b6b1d72247ee390b471b68cdedd632b8530e88e5

Modified Files
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src/tutorial/advanced.source | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)