Add a package declaration to the Java extern example.
This makes it more clear where the javaimpl "class" attribute comes from.
@@ -74,6 +74,9 @@ The most common way to do this is to implement the function as a `public static`
method in some Java class.
```java
+package com.google.apps.framework.modulesets.soyplugins;
+
+...
public class StaticRuntime {
