Fix grammar: "consist" -> "consists"

Addressing review comment by Hans.

chapters/interface.tex

@@ -217,7 +217,7 @@ \section{Interface or Type}\doublelabel{interface-or-type}
 In a class all references to elements of that class should be limited to their constraining interface.
 
 \begin{nonnormative}
-The \emph{constraining interface} constsist of only the public elements, and if the declaration is replaceable the element is limited to the constraining interface.
+The \emph{constraining interface} constsists of only the public elements, and if the declaration is replaceable the element is limited to the constraining interface.
 \end{nonnormative}
 
 \begin{nonnormative}