convert INABIAF headers in README.md to a Jargon File quote

We introduce a bit of fun, while specifying that something
is "supposed to happen".

NOTE: There is no need to change the INABIAF STATUS.  The use
there is perfectly fine.

1990/theorem/README.md

@@ -48,13 +48,18 @@ y1 - initial value  (y(x1) == y1)
 ./theorem y 0 1 0.1 1
 ```
 
-### INABIAF - it's not a bug it's a feature! :-)
-
 If the two args passed to `fibonacci` and `theorem_bkp` equals 0 it will
 print 0 over and over again in an infinite loop. In this case it should not be
 fixed (another condition where this occurred was fixed as it affected usability
 of the program).
 
+This is supposed to happen.  As is written in the
+[The Jargon File](http://catb.org/jargon/html/F/feature.html):
+
+```
+That's not a bug, that's a feature.
+```
+
 BTW: why can't the fix:
 
 ```c