Fix the versionadded indentation in exec_module doc (GH-8719)

diff --git a/Doc/library/importlib.rst b/Doc/library/importlib.rst
index 74a73fd..6f4da11 100644
--- a/Doc/library/importlib.rst
+++ b/Doc/library/importlib.rst
@@ -770,7 +770,7 @@
 
        Concrete implementation of :meth:`Loader.exec_module`.
 
-      .. versionadded:: 3.4
+       .. versionadded:: 3.4
 
     .. method:: load_module(fullname)