Beta decay is a type of radioactive decay in which a beta ray is emitted from an atomic nucleus, and in this type of decay, one proton in the nucleus is transformed into a neutron and vice versa, therefore for Platinum 201 we have:
201/78 Pt (this is the platinum-201) -> 200/79 Au + antineutrino + electron
This is the formula for a Beta minus decay, where an electron is released