Fixed javadoc mistake.

bundles/org.eclipse.core.runtime/src/org/eclipse/core/runtime/preferences/PreferenceModifyListener.java

diff --git a/bundles/org.eclipse.core.runtime/src/org/eclipse/core/runtime/preferences/PreferenceModifyListener.java b/bundles/org.eclipse.core.runtime/src/org/eclipse/core/runtime/preferences/PreferenceModifyListener.java
index 1e04f6c..7149a3a 100644
--- a/bundles/org.eclipse.core.runtime/src/org/eclipse/core/runtime/preferences/PreferenceModifyListener.java
+++ b/bundles/org.eclipse.core.runtime/src/org/eclipse/core/runtime/preferences/PreferenceModifyListener.java
@@ -16,7 +16,7 @@
  * the preference service via XML and the <code>org.eclipse.core.runtime.preferences</code>
  * extension point.
  * <p>
- * Clients may implement this interface.
+ * Clients may subclass this type.
  * </p>
  * 
  * @since 3.1