I suspect this is platform related, the SRX300 series is NOT listed on any port mirroring kb that I can find.
You can submit a kb article feedback on the right side of that page. The document owner will get your note that the procedure does not work on the SRX300 and open a case to update the documentation. I've used this before to get documentation corrected or updated. But you do need to be patient as this goes through the normal updates workflow.
If no one here has done this on an SRX300 yet, you will need to open a ticket then for a quicker answer.