remove txt extension from license

README.md

@@ -124,7 +124,7 @@ whose full text can be found at: http://opendatacommons.org/licenses/pddl/1.0/
 See also the human readable summary at:
 https://opendatacommons.org/licenses/pddl/summary/
 
-Please see the [LICENSE.txt](./LICENSE.txt) file for details.
+Please see the [LICENSE](./LICENSE) file for details.
 
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