Add space after rsh arg formatting

This was causing borgbackup to emit args with "create"
appended to the rsh string instead of being its own
entry after being split by shlex:

    [..., "ssh -i /mykeycreate", "--json", ...]
to -->
    [..., "ssh -i /mykey", "create", "--json", ...]

src/common.rs

@@ -269,7 +269,7 @@ impl From<&CommonOptions> for String {
         let mut s = String::new();
 
         if let Some(rsh) = &value.rsh {
-            s = format!("{s} --rsh {}", shell_escape(rsh));
+            s = format!("{s} --rsh {} ", shell_escape(rsh));
         }
 
         if let Some(remote_path) = &value.remote_path {