Added map constraint for parameter content property

Author: darrelmiller

versions/3.0.md

@@ -993,7 +993,7 @@ When `example` or `examples` are provided in conjunction with the `schema` objec
 
 Field Name | Type | Description
 ---|:---:|---
-<a name="parameterContent"></a>content | Map[`string`, [Media Type Object](#mediaTypeObject)] | An map containing the representations for the parameter. The key is the media type and the value is used to describe it.
+<a name="parameterContent"></a>content | Map[`string`, [Media Type Object](#mediaTypeObject)] | An map containing the representations for the parameter. The key is the media type and the value is used to describe it.  The map MUST only contain one entry.
 
 ##### Style Values