Fix javadoc in example

[idelpivnitskiy]

Motivation:

./gradlew javadoc command fails due to some javadoc rules violations
in examples.

Modifications:

• Don't use self-closing tags;
• Fix a link attribute for a tag;
• Use <pre> + {@code ...} to not worry about manual chars escape;

Result:

./gradlew javadoc command is happy.

[bondolo]

Even though we do not publish the examples JavaDocs should we perhaps generate it during all builds (it won't take much time, probably < 1second) to ensure that it remains compliant?

[idelpivnitskiy]

@bondolo makes sense! We can handle it separately. I've created an issue #1715 to keep track of it.