Fix typo in equation

Author: davidlmobley

paper/basic_training.tex

@@ -204,7 +204,7 @@ \subsubsection{Key concepts}
 \end{itemize}
 
 One of the main objectives of molecular simulations is to estimate/predict thermodynamic behavior of real systems as observed in the laboratory.
-Typically this means we are interested in macroscopic systems, consisting of $10^23$ particles or more (i.e. at least several moles of particles). 
+Typically this means we are interested in macroscopic systems, consisting of $10^{23}$ particles or more (i.e. at least several moles of particles). 
 But properties of interest include not only macroscopic, bulk thermodynamic properties, such as density or heat capacity,
 but also microscopic properties like specific free energy differences associated with, say, changes in the conformation of a molecule.
 For this reason, it is important to understand key concepts in thermodynamics, such as temperature, pressure, entropy, internal energy, various forms of free energy, and the relationships between them.