There isn't currently a merge field for "today's date". You can add this into any text box as required within your Document. Alternatively, a couple of workarounds that may be temporary solutions for you:

Use the Document Created At field instead. This of course will only work if you're sending the Document on the same date you created it.

Another option is to create a Date booking Custom Field, where you can select the date and using the merge field, this information will then pull into the Document.