The sūtra is mentioned in the first prakaraṇa named Agnicayāḥ itself. Also, this is the correct one -
> dīrghacaturaśrasyākṣṇayārajjuḥ pārśvamāni tiryaṅmānī ca yatpṛthagbhūte kurutastadubhayaṁ karoti
The pdf of Baudhāyana śulba sūtra is available here
https://hinduonline.co/Scriptures/Sutra/BaudhayanaShulbaSutra.html
And the same sūtra is mentioned verbatim in Hiraṇyakeśi śulbasūtra as well.