setup.py: Fix description content-type to markdown

Otherwise, the upload-to-pypi build complains about it [1]

[1] https://jenkins.softwareheritage.org/job/DLDCVS/job/pypi-upload/12/console

setup.py

@@ -40,7 +40,7 @@ setup(
     name="swh.loader.cvs",
     description="Software Heritage CVS Loader",
     long_description=long_description,
-    long_description_content_type="text/x-rst",
+    long_description_content_type="text/markdown",
     python_requires=">=3.7",
     author="Software Heritage developers",
     author_email="swh-devel@inria.fr",