Add a reference to vector types example in vector-heads-tails.py (#988)

Author: michaelgrund

examples/gallery/line/vector-heads-tails.py

@@ -25,8 +25,9 @@
 
 In the following we use the :meth:`pygmt.Figure.plot` method to plot vectors
 with individual heads and tails. We must specify the modifiers (together with
-the vector type, here ``v``) by passing the corresponding shortcuts to the
-``style`` parameter.
+the vector type, here ``v``, see also
+:doc:`Vector types documentation </gallery/line/vectors>`.) 
+by passing the corresponding shortcuts to the ``style`` parameter.
 
 """