Center the cartopy text in the logo

[philippemiron]

I updated the script that generates the logo in the example, and used it to output a new png icon.

Rationale

Fix issue 1465. The code to centered the logo is here. I updated the offset (longitude, latitude) values in the script.

Implications

The logo is now centered and the png and logo.py script output match.

[philippemiron]

I have submitted the CLA.

[CLAassistant]

All committers have signed the CLA.