Coloc_2: attempt to fix Travis javadoc error

Locally, javadoc:javadoc passes on my macOS 10.12 system.
However, the Travis CI build barfs with:

[ERROR] /home/travis/build/fiji/Colocalisation_Analysis/src/main/java/sc/fiji/coloc/Coloc_2.java:438: error: reference not found
[ERROR] * {@link #colocalise(Img, Img, BoundingBox, Img, List)} with {@code null} for

The BoundingBox class is an inner class of sc.fiji.coloc.Coloc_2;
maybe that is the problem. We'll see if qualifying it helps.