[mm] Fix wrong pin for Probe3 in README
[mm] Fix wrong pin for Probe3 in README

CapnBry/HeaterMeter,CapnBry/HeaterMeter,CapnBry/HeaterMeter,CapnBry/HeaterMeter,CapnBry/HeaterMeter,CapnBry/HeaterMeter,CapnBry/HeaterMeter