[skipci] Fix the syntax of a call to 'twine'

Author: ziotom78

RELEASES.md

@@ -22,7 +22,7 @@
 -   Upload the `.tar.gz` and `.whl` files to the PyPI Test server:
 
     ```
-    twine upload --repository-url https://test.pypi.org/legacy/ dist/litebird_sim_*
+    twine upload --repository testpypi dist/litebird_sim_*
     ```
 
 -   Check that everything looks right by opening the URL shown by Twine