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Think about it in terms of how many different molecules are affecting the rate at which the reaction occurs. I don't know whether or not you've discussed rate laws in your chemistry class or not yet, but if you understand them, the molecularity of a reaction becomes much more apparent.
In this case, you have NO decomposing to N2 and O2. Though it is necessary to give NO a coefficient of 2 to preserve stoichiometric balance, it is still only the amount of NO in the system that determines the speed at which the reaction proceeds. In short, because the concentration of only one type of molecule is governing the rate of the reaction, this is a unimolecular reaction.
If you're familiar with rate laws, think about it this way; you know that the rate law formula for this equation is only going to contain NO. The coefficient of 2 will show up as an exponent in the rate law -- it does not add another reactant to the formula. A reaction whose rate law equation contains only a single reactant is known as a first-order reaction, and this will be unimolecular.
I can elaborate if you like, hope this helped. If the rate law stuff was too much, just keep in mind that the molecularity is governed by the number of reactants whose varying concentrations can affect the rate at which the reaction in question occurs.
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