Update changelog: note that forall_inst was changed

spec/changelog.tex

@@ -16,6 +16,9 @@ Breaking changes:
   \item Add the sort to all variables in contexts.  Before, the context
         of a bind could be \texttt{(x S) (:= y x)}.  Now it must
         be \texttt{(x S) (:= (y S) x)}.
+  \item The arguments for \proofRule{forall_inst} have been changed to
+        no longer take the shape of bindings using \texttt{(:= x c)}.
+        Instead, the list of instatiation terms must follow the variable order.
 \end{itemize}
 
 \noindent