Fix error in AbstractResource#contentLength Javadoc

Issue: SPR-8734

org.springframework.core/src/main/java/org/springframework/core/io/AbstractResource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2010 the original author or authors.
+ * Copyright 2002-2011 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -108,7 +108,7 @@ public abstract class AbstractResource implements Resource {
 	}
 
 	/**
-	 * This implementation checks the timestamp of the underlying File,
+	 * This implementation checks the length of the underlying File,
 	 * if available.
 	 * @see #getFile()
 	 */