fix typo in markup

Doc/lib/libcodecs.tex

@@ -398,7 +398,7 @@ order to be compatible to the Python codec registry.
   Decodes data from the stream and returns the resulting object.
 
   \var{chars} indicates the number of characters to read from the
-  stream. \function{read()} will never return more than \vars{chars}
+  stream. \function{read()} will never return more than \var{chars}
   characters, but it might return less, if there are not enough
   characters available.