Fix typos in subtype figure caption

chapters/interface.tex

@@ -121,7 +121,7 @@ \section{The Concepts of Type, Interface and Subtype}\label{the-concepts-of-type
   \begin{center}
     \includegraphics{subtype}
   \end{center}
-  \caption{The type \lstinline!R! can be defined as the set o record values containing \lstinline!x! and \lstinline!b!.  The subtype \lstinline!R2! is the subset of value that all contain
+  \caption{The type \lstinline!R! can be defined as the set of record values containing \lstinline!x! and \lstinline!b!.  The subtype \lstinline!R2! is the subset of values that all contain
   \lstinline!x!, \lstinline!b!, and \lstinline!y!.}
 \end{figure}
 \end{example}