Small change to input doc for rmg familes
Small change to input doc for rmg familes

Just made it a little clearer that the 'default' option is just a
string, while multiple Families are designated as a list of strings

pierrelb/RMG-Py,enochd/RMG-Py,nickvandewiele/RMG-Py,enochd/RMG-Py,chatelak/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py,comocheng/RMG-Py,nickvandewiele/RMG-Py