Corrected docs

tm/scribblings/tm.scrbl

@@ -254,7 +254,7 @@ This step also involves building the label table, determining the starting line
 }
 
 @subsection{The tape}
-The tape is essentially implemented using two queues.
+The tape is essentially implemented using two stacks.
 
 @defstruct*[tape ([before list?] [after list?] [pos integer?] [leftmost-pos integer?])]{
 A structure type representing a Turing machine tape.