Fix context:property-placeholder XSD type hierarchy

context:property-placeholder extends 'propertyPlaceholder' type
defintion once again.  This relationship was inadvertently removed in
3.1 M1, and the effect was that XML tooling would raise errors on
use of attributes like 'location'.

The updated schema has also been published to

    http://www.springframework.org/schema/context/spring-context-3.1.xsd

Issue: SPR-8037

org.springframework.context/src/main/resources/org/springframework/context/config/spring-context-3.1.xsd

@@ -90,7 +90,7 @@
 		<xsd:annotation>
 			<xsd:documentation><![CDATA[
 	Activates replacement of ${...} placeholders, resolved against the specified properties file or
-	Properties object (if any). Defines an PropertySourcesPlaceholderConfigurer within the context.
+	Properties object (if any). Defines a PropertySourcesPlaceholderConfigurer within the context.
 	For backward compatibility with versions earlier than Spring 3.1, a PropertyPlaceholderConfigurer will be
 	registered if the 'system-properties-mode' attribute has been explicitly assigned a value.
 			]]></xsd:documentation>
@@ -101,6 +101,11 @@
 				</tool:annotation>
 			</xsd:appinfo>
 		</xsd:annotation>
+		<xsd:complexType>
+			<xsd:complexContent>
+				<xsd:extension base="propertyPlaceholder" />
+			</xsd:complexContent>
+		</xsd:complexType>
 	</xsd:element>
 
 	<xsd:element name="property-override">