src/doc/en/developer/portability_testing.rst: Fix markup

Author: Matthias Koeppe

src/doc/en/developer/portability_testing.rst

@@ -987,8 +987,7 @@ errors or warnings issued during the build.
 
 In addition to these automatic runs in our main repository, all Sage
 developers can run the same tests on GitHub Actions in their personal
-forks of the Sage repository. To prepare this, `enable GitHub Actions
-  <https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository#managing-github-actions-permissions-for-your-repository>`_
+forks of the Sage repository. To prepare this, `enable GitHub Actions <https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository#managing-github-actions-permissions-for-your-repository>`_
 in your fork of the Sage repository.
 
 As usual we assume that ``origin`` is the name of the remote