Update pet_okubo_weiss.py

change sign in formula

Author: AntSimi

examples/06_grid_manipulation/pet_okubo_weiss.py

@@ -2,7 +2,7 @@
 Get Okubo Weis
 ==============
 
-.. math:: OW = S_n^2 + S_s^2 + \omega^2
+.. math:: OW = S_n^2 + S_s^2 - \omega^2
 
 with normal strain (:math:`S_n`), shear strain (:math:`S_s`) and vorticity (:math:`\omega`)