update XML namespace documentation

The XML namespace documentation was changed to better reflect the
expected value.  We changed the value type from a URL to an absolute
URI and the wording was that this is a strict requirement and not a
suggestion.

Fixes OAI#1084

Author: whitlockjc

versions/3.0.md

@@ -3002,7 +3002,7 @@ See examples for expected behavior.
 Field Name | Type | Description
 ---|:---:|---
 <a name="xmlName"></a>name | `string` | Replaces the name of the element/attribute used for the described schema property. When defined within `items`, it will affect the name of the individual XML elements within the list. When defined alongside `type` being `array` (outside the `items`), it will affect the wrapping element and only if `wrapped` is `true`. If `wrapped` is `false`, it will be ignored.
-<a name="xmlNamespace"></a>namespace | `string` | The URL of the namespace definition. Value SHOULD be in the form of a URL.
+<a name="xmlNamespace"></a>namespace | `string` | The URI of the namespace definition. Value MUST be in the form of an absolute URI.
 <a name="xmlPrefix"></a>prefix | `string` | The prefix to be used for the [name](#xmlName).
 <a name="xmlAttribute"></a>attribute | `boolean` | Declares whether the property definition translates to an attribute instead of an element. Default value is `false`.
 <a name="xmlWrapped"></a>wrapped | `boolean` | MAY be used only for an array definition. Signifies whether the array is wrapped (for example, `<books><book/><book/></books>`) or unwrapped (`<book/><book/>`). Default value is `false`. The definition takes effect only when defined alongside `type` being `array` (outside the `items`).