Fixed install generated $dolibarr_main_url_root_alt path

We now use relative paths.
So let's remove the absolute part from the install generated configuration file.

htdocs/install/etape1.php

@@ -812,7 +812,7 @@ function write_conf_file($conffile)
         fputs($fp, '$dolibarr_main_document_root=\''.str_replace("'","\'",($main_dir)).'\';');
         fputs($fp,"\n");
 
-        fputs($fp, $main_use_alt_dir.'$dolibarr_main_url_root_alt=\''.str_replace("'","\'",($main_url."/".$main_alt_dir_name)).'\';');
+        fputs($fp, $main_use_alt_dir.'$dolibarr_main_url_root_alt=\''.str_replace("'","\'",("/".$main_alt_dir_name)).'\';');
         fputs($fp,"\n");
 
         fputs($fp, $main_use_alt_dir.'$dolibarr_main_document_root_alt=\''.str_replace("'","\'",($main_dir."/".$main_alt_dir_name)).'\';');