Don't set box-forced in Cartopy example. (#3273)

It is deprecated in Matplotlib 2.2, removed in 3.1, and appears to have
no effect on the result.

Author: QuLogic

doc/gallery/plot_cartopy_facetgrid.py

@@ -41,6 +41,6 @@
     ax.set_extent([-160, -30, 5, 75])
     # Without this aspect attributes the maps will look chaotic and the
     # "extent" attribute above will be ignored
-    ax.set_aspect("equal", "box-forced")
+    ax.set_aspect("equal")
 
 plt.show()