docs: update example in readme

[usefulthink]

fixes #573

[deAtog]

You should have included the examples I provided for setZoom and setCenter as they demonstrate how to respond to changes in the zoom and center properties passed to the component. Without those, the center and zoom will not be updated if those properties change.

[usefulthink]

@deAtog that is a good point, but I wanted to keep the example as simple as possible.

Also, handling properties like that can get quite complicated if you want to do it right, so my recommendation at that point would be to use our @vis.gl/react-google-maps library instead of writing your own.