The headers field is an element, not an attribute

The field "headers" was defined as an attribute but it's a complex type
so it's an element instead.

See: #272

Resources/config/schema/monolog-1.0.xsd

@@ -88,7 +88,7 @@
         <xsd:attribute name="content-type" type="xsd:string" />
         <xsd:attribute name="webhook-url" type="xsd:string" />
         <xsd:attribute name="slack-team" type="xsd:string" />
-        <xsd:attribute name="headers" type="headers" />
+        <xsd:element name="headers" type="headers" />
     </xsd:complexType>
 
     <xsd:simpleType name="level">