Correct file extension used for CC0

f3eeda4 (Adding license, 2022-10-14) added a copy of the plain text
version of CC0 [1]. That commit mistakenly used the .md file extension
for the plain text file that it created.

GitHub automatically assumes that .md files contain Markdown [2]. As a
result, the plain text version of CC0 would get rendered as Markdown
which made it significantly harder to read. The lists inside sections 1
and 4 would be rendered as amorphous paragraphs.

This commit changes the file extension to .txt to prevent GitHub from
making CC0 harder to read.

[1]: <https://creativecommons.org/publicdomain/zero/1.0/legalcode.txt>
[2]: <b2230a2959/README.md (markups)>

README.md

@@ -106,7 +106,7 @@ this larger namespace.  But, we don't want to be so general that the spec become
 
 ## License
 
-The "podcast" namespace is in the public domain under a CC0 v1.0 [license](LICENSE.md) so as to remove all barriers to adoption, development and contribution.
+The "podcast" namespace is in the public domain under a CC0 v1.0 [license](LICENSE.txt) so as to remove all barriers to adoption, development and contribution.
 
 
 <br><br>