Fix heading level of 'Try' section

It was agreed to be '## Try:' but there were many files where it was
'### Try:'. This has been corrected.

1990/tbr/README.md

@@ -52,7 +52,7 @@ or ctrl-c (or kill it from another terminal :-) ). But why does it not work with
 just `exit` or `logout`? Hint: see the authors' remarks.
 
 
-### Try:
+## Try:
 
 ```sh
 ./tbr