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.

1993/dgibson/README.md

@@ -16,7 +16,7 @@ where:
 `datafile` is the initial pattern (default: [dgibson.data1](dgibson.data1))
 
 
-### Try:
+## Try:
 
 ```sh
 ./try.sh