change CODE URL from FTP to HTTP

See issue report on the old prefactor github page: https://github.com/lofar-astron/prefactor/issues/297

as well as commit to master: https://github.com/lofar-astron/prefactor/commit/73809818e7cfac171ce91a12024b7ca10552eee2

docs/source/target.rst

@@ -321,7 +321,7 @@ A comprehensive explanation of the baseline selection syntax can be found `here`
 
 *RMextract settings*
 
-- ``ionex_server``: URL of the *IONEX* server (default: ``"ftp://ftp.aiub.unibe.ch/CODE/"``)
+- ``ionex_server``: URL of the *IONEX* server (default: ``"http://ftp.aiub.unibe.ch/CODE/"``)
 - ``ionex_prefix``: the prefix of the *IONEX* files (default: ``CODG``)
 - ``proxy_server``: specify URL or IP of proxy server if needed
 - ``proxy_port``: port of proxy server if needed