fixup! docs: borg serve: recommend using a simple shell (closes #3818)

Co-authored-by: TW <[email protected]>

docs/usage/serve.rst

@@ -77,7 +77,7 @@ If you then run borg commands with ``--lock-wait 600``, this gives sufficient ti
 You may, of course, modify the timeout values demonstrated above to values that suit your environment and use case.
 
 When the client is untrusted, it is a good idea to set the backup
-user's shell to a simple implementation (The example /bin/sh may or may
+user's shell to a simple implementation (``/bin/sh`` is only an example and may or may
 not be such a simple implementation)::
 
   chsh -s /bin/sh BORGUSER