Add the "-O" flag to scp commands to force scp instead of sftp

This commit simply adds the "-O" flag to the scp commands used
in the offboarding guide, which is now required so that it uses
scp instead of trying sftp (which will result in an error).

Fixes #592

docs/admin/deployment/offboarding.rst

@@ -84,13 +84,13 @@ SSH key, you should rotate the key in the following manner.
 
     .. code:: sh
 
-      scp /home/amnesia/.ssh/newkey.pub scp://app
+      scp -O /home/amnesia/.ssh/newkey.pub scp://app
 
     and
 
     .. code:: sh
 
-      scp /home/amnesia/.ssh/newkey.pub scp://mon
+      scp -O /home/amnesia/.ssh/newkey.pub scp://mon
 
 
 #.  Add this key to the list of authorized keys.