protoc-java: Fix broken javadoc generation

[ST-DDT]

protoc-3.10.0 generates broken javadocs for repeated enum values:

/**
 * <pre>
 * A list of types.
 * </pre>
 * <code>repeated .foo.bar.ItemType types = 1;</code>
 * @param value The types to add.
 */
public int getTypesCount() {
    return types_.size();
}

which causes an error during javadoc generation.

This PR fixes that issue by using the correct parameter.
The error was accidentally added by me in #6231

Maybe this fix could be added to a 3.10.1 release.

[acozzette]

Thanks, @ST-DDT.