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

This is a follow-on patch to 92c12e46.  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@inbox.ru

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

Backpatch-through: 9.6

src/tutorial/advanced.source

@@ -23,7 +23,7 @@
 CREATE TABLE cities (
 	name		text,
 	population	float8,
-	altitude	int		-- (in ft)
+	elevation	int		-- (in ft)
 );
 
 CREATE TABLE capitals (
@@ -42,17 +42,17 @@ SELECT * FROM cities;
 SELECT * FROM capitals;
 
 -- You can find all cities, including capitals, that
+-- are located at an elevation of 500 ft or higher by:
 
-
+SELECT c.name, c.elevation
 FROM cities c
-FROM cities c
+WHERE c.elevation > 500;
 
 -- To scan rows of the parent table only, use ONLY:
 
-
+SELECT name, elevation
 FROM ONLY cities
-FROM ONLY cities
+WHERE elevation > 500;
 
 
 -- clean up (you must remove the children first)