update comment CDS Service attributes to SHALL (fixes #215)

Author: brettmarquard

docs/specification/1.0.md

@@ -46,7 +46,7 @@ Field | Description
 ----- | ---------
 `services` | *array*. An array of **CDS Services**
 
-Each CDS Service is described by the following attributes.
+Each CDS Service SHALL be described by the following attributes.
 
 Field | Optionality | Type | Description
 ----- | ----- | ----- | ---------