Replace references to LICENSE.md to LICENSE.txt

Author: maarten-ic

docs/source/index.rst

@@ -77,5 +77,5 @@ Manual
 LICENSE
 -------
 
-.. literalinclude:: ../../LICENSE.md
+.. literalinclude:: ../../LICENSE.txt
    :language: text

pyproject.toml

@@ -15,7 +15,7 @@ authors = [
 description = "Pythonic wrappers for the IMAS Access Layer"
 readme = {file = "README.md", content-type = "text/markdown"}
 requires-python = ">=3.7"
-license = {file = "LICENSE.md"}
+license = {file = "LICENSE.txt"}
 classifiers = [
     "Development Status :: 3 - Alpha",
     "Environment :: Console",