Add import default backend (#4061)

* Add import default backend

* Revert blank line in
diff --git a/docs/hazmat/primitives/asymmetric/rsa.rst b/docs/hazmat/primitives/asymmetric/rsa.rst
index 607cebb..4634dd4 100644
--- a/docs/hazmat/primitives/asymmetric/rsa.rst
+++ b/docs/hazmat/primitives/asymmetric/rsa.rst
@@ -64,6 +64,7 @@
 
 .. code-block:: pycon
 
+    >>> from cryptography.hazmat.backends import default_backend
     >>> from cryptography.hazmat.primitives import serialization
 
     >>> with open("path/to/key.pem", "rb") as key_file: