Update geo_tile_source.py
Update documentation on using tile providers with the `get_provider()` method.
from bokeh.plotting import figure, show, output_file
-from bokeh.tile_providers import CARTODBPOSITRON
+from bokeh.tile_providers import get_provider, Vendors
output_file("tile.html")
+tile_provider = get_provider(Vendors.CARTODBPOSITRON)
+
# range bounds supplied in web mercator coordinates
p = figure(x_range=(-2000000, 6000000), y_range=(-1000000, 7000000),
x_axis_type="mercator", y_axis_type="mercator")
-p.add_tile(CARTODBPOSITRON)
+p.add_tile(tile_provider)
show(p)
