Update README.md

README.md

@@ -16,7 +16,7 @@ Developer repository: https://github.com/xmaptools/XMapTools_Public
 
 PTtoolbox (by J. Laughton): https://github.com/JoshuaLaughton/PTtoolbox
 
-<a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-sa/4.0/88x31.png" /></a><br />This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">Creative Commons Attribution-ShareAlike 4.0 International License</a>.
+<a rel="license" href="https://www.gnu.org/licenses/gpl-3.0.en.html"><img alt="GNU-GPLv3" style="border-width:0" src="https://www.gnu.org/graphics/agplv3-with-text-162x68.png" /></a><br />This work is licensed under a <a rel="license" href="https://www.gnu.org/licenses/gpl-3.0.en.html">GPL-3.0 license</a>.
 
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 <a href="https://ec.europa.eu/info/research-and-innovation/funding/funding-opportunities/funding-programmes-and-open-calls/horizon-europe_en" target="_blank"><img src="https://user-images.githubusercontent.com/54409312/168419191-401f1d2c-014f-4640-9edc-cfa6f3832a40.png" height="70"/></a>