Move velo_arrow_ellipse.py under "Seismology and Geodesy" category

Co-Authored-By: Dongdong Tian <seisman.info@gmail.com>

Author: weiji14

examples/gallery/symbols/velo_arrow_ellipse.py renamed to examples/gallery/seismology/velo_arrow_ellipse.py

@@ -3,12 +3,11 @@
 ---------------------------------------
 
 The :meth:`pygmt.Figure.velo` method can be used to plot mean velocity arrows
-and confidence ellipses.
-The example below plots red velocity arrows with light-blue confidence ellipses
-outlined in red with the east_velocity x north_velocity used for the station names.
-Note that the velocity arrows are scaled by 0.2 and the 39% confidence limit
-will give an ellipse
-which fits inside a rectangle of dimension east_sigma by north_sigma.
+and confidence ellipses. The example below plots red velocity arrows with
+light-blue confidence ellipses outlined in red with the east_velocity x
+north_velocity used for the station names. Note that the velocity arrows are
+scaled by 0.2 and the 39% confidence limit will give an ellipse which fits
+inside a rectangle of dimension east_sigma by north_sigma.
 """
 
 import pandas as pd