fix a document

doc/engineering-reference/src/simulation-models-encyclopedic-reference-005/occupant-thermal-comfort.tex

@@ -1217,7 +1217,7 @@ \subsubsection{Description of the model and algorithm}\label{description-of-the-
 
 This equation is only applicable for \(v_{r} < 0.2 m/s\) . And the subject’s metabolic rate and clothing level should be kept below 1.3 met and 0.7 clo, respectively. PPD at ankle draft will be set to -1.0 if if these conditions are not met.
 
-It should be noted that the metrics only measures the predicted percentage dissatisfied on the sensation of draft. PPD_AD levels at various air speeds and thermal sensations are depicted in Figure~\ref{fig:ankle-draft-ppd}
+It should be noted that the metrics only measures the predicted percentage dissatisfied on the sensation of draft. \(PPD_{AD}\) levels at various air speeds and thermal sensations are depicted in Figure~\ref{fig:ankle-draft-ppd}
 
 \begin{figure}[hbtp]
 \centering