MNT: Update op parameter to predicate

Apparently this was deprecated in 0.14, but we never noticed the warning
since it's only in the generated docs.

examples/plots/nhc_wind_probabilities.py

@@ -57,7 +57,7 @@
 # cities we selected above. Geopandas provides a spatial join method, which merges the two
 # GeoDataFrames and can tell us which wind speed probability polygon each of our city points
 # lies within. That information is stored in the 'PERCENTAGE' column below.
-cities = geopandas.sjoin(cities, wind_data, how='left', op='within')
+cities = geopandas.sjoin(cities, wind_data, how='left', predicate='within')
 cities
 
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