re-wording

ch02.adoc

@@ -289,7 +289,7 @@ Aggregation variables may be used as any kind of variable (data variable, coordi
 Any text applying to a variable in the CF conventions applies in exactly the same way to an aggregation variable in the same role; and any reference to the dimensions or data of a variable applies to the aggregated dimensions or aggregated data, respectively, of an aggregation variable.
 For instance:
 
-* The dimension of a coordinate variable of an aggregation data variable must be one of the aggregated dimensions of the aggregation data variable.
+* The dimension of a coordinate variable of an aggregation data variable must be included as one of the aggregated dimensions of the aggregation data variable.
 
 * The name of an aggregation coordinate variable (which is a scalar) must
 be the same as the name of its single aggregated dimension (identified by its **`aggregated_dimensions`** attribute), just as the name of a coordinate variable (which is one-dimensional) must be the same as the name of its single