correct set command example in fix property/atom

fix prop all property/atom mol

variable cluster atom ((id-1)/10)+1

set id * mol v_cluster :pre

set atom * mol v_cluster :pre

The "atom-style variable"_variable.html will create values for atoms

with IDs 31,32,33,...40 that are 4.0,4.1,4.2,...,4.9.  When the