What is the displayed formula for methylpropene?
"Propene" is what defines the main chain of three atoms so that the maximum number of carbon atoms in any other chain cannot exceed this number. Hence your proposition that


  there would be a one-carbon side chain on the first carbon


is wrong as in this case there would be a 4-membered chain of either 1-butene or 2‐butene:



In order to comply with the methylpropene name, one can only attach methyl group to the second carbon, and not to the terminal one:



To sum it up, your textbook is correct.