Update relational-data.Rmd (minor typo) (hadley#641)

Row 371 : In table=`airports`, the variable representing longitude is not `long` ...it is `lon`

relational-data.Rmd

@@ -368,7 +368,7 @@ So far, the pairs of tables have always been joined by a single variable, and th
     variables from `x` will be used in the output.
 
     For example, if we want to draw a map we need to combine the flights data
-    with the airports data which contains the location (`lat` and `long`) of
+    with the airports data which contains the location (`lat` and `lon`) of
     each airport. Each flight has an origin and destination `airport`, so we
     need to specify which one we want to join to: