Fixed typo.

docs/usage.rst

@@ -66,7 +66,7 @@ General:
         When set, use the given filename as INI_-style logging configuration.
     BORG_RSH
         When set, use this command instead of ``ssh``. This can be used to specify ssh options, such as
-        a custom identity file ``ssh -i /path/to/private/key``. See ``ssh man`` for other options.
+        a custom identity file ``ssh -i /path/to/private/key``. See ``man ssh`` for other options.
     TMPDIR
         where temporary files are stored (might need a lot of temporary space for some operations)