[FrameworkBundle] Fix XSD definition

Author: fabpot

Resources/config/schema/symfony-1.0.xsd

@@ -578,7 +578,7 @@
 
     <xsd:complexType name="mailer_envelope">
         <xsd:sequence>
-            <xsd:element name="sender" type="xsd:string" />
+            <xsd:element name="sender" type="xsd:string" minOccurs="0" maxOccurs="1" />
             <xsd:element name="recipients" type="xsd:string" minOccurs="0" maxOccurs="unbounded" />
         </xsd:sequence>
     </xsd:complexType>