This isn't true, you can shrink a transaction log file that is in a log shipping configuration without affecting the log shipping.

However, I'd only do it if the log has grown on a one off disproportionate amount (e.g. an excessively large transaction, or if for any reason your log backup job failed for any period causing unnecessary log growth), otherwise the log will simply grow back to that size.

If that's the case, then you're better off performing your log backups more frequently.