example: Fixes in the presentation of TBN/TBF.

example/example.org

@@ -909,7 +909,7 @@ tab
 
    Here's how you can read this TBF from an Org-mode table:
    #+NAME: simple-tbf/state
-   | a | b | f |
+   | a | b | θ |
    | 1 | 1 | 1 |
 
    #+BEGIN_SRC racket :results output drawer :var simple-tbf/state=munch-sexp(simple-tbf/state)
@@ -926,7 +926,7 @@ tab
    =read-org-tbfs/state= can also read multiple TBFs at once:
 
    #+NAME: simple-tbfs/state
-   |  a | b | f |
+   |  a | b | θ |
    |  1 | 1 | 1 |
    | -2 | 1 | 0 |