Update spatial_relationships_exercises.rst

Using a subquery or a MULTILINESTRING instead of the suggested MULTILINESTRING->LINESTRING simplification gives different and expected results.
There seems to be a difference between multilinestring and linestring having the same coordinates!

Author: ThomasPrioul

postgis-intro/sources/en/spatial_relationships_exercises.rst

@@ -83,14 +83,15 @@ Exercises
     FROM nyc_streets 
     WHERE ST_DWithin(
       geom, 
-      ST_GeomFromText('LINESTRING(586782 4504202,586864 4504216)', 26918),
+      (SELECT geom FROM nyc_streets WHERE name = 'Atlantic Commons'),
       0.1
     );
     
   ::
   
            name
       ------------------
+       S Oxford St
        Cumberland St
        Atlantic Commons