Fix tight_layout warning on cartopy facetgrid docs example

[shoyer]

Per the fix in #4032, I'm pretty sure we will soon start seeing a warning message printed on ReadTheDocs in Cartopy FacetGrid example:
http://xarray.pydata.org/en/stable/plotting.html#maps

This would be nice to fix for users, especially because it's likely users will see this warning when running code outside of our documentation, too.

[stale]

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