[composer][SampleCode]Update code mark for sample code snippet in Javadoc

[summer-ji-eng]

We will use go/java-practices/javadoc recommended format for block-style code excerpts in Javadoc that is:

/**
 * <p>Paragraph.
 *
 * <pre>{@code
 * Your code here.
 * Can include <angle brackets>.
 * You can even include snippets that contain annotations, e.g.:
 * @Override public String toString() { ... }
 * }</pre>
 *
 * <p>Following paragraph.
 */

[miraleung]

Note that @ will need special handling, but feel free to tackle that separately.

For now, there are no sample code example use @.
I've added comment note to handle this case if future needs, LMK if you have further concern.