Fix an erroneously moved line

Author: shym

doc/stm/index.mld

@@ -408,9 +408,9 @@ end = struct
 
   let add a t =
     Mutex.lock t.mutex;
+    if not (mem_non_lock a t) then begin
       t.content  <- a :: t.content;
       t.cardinal <- t.cardinal + 1;
-    if not (mem_non_lock a t) then begin
     end;
     Mutex.unlock t.mutex