Change heading level of 'Original code'

In the case where we recommend trying the alternate code first, the
lower section is 'Original code' but this was at heading level 3 when
discussion suggests 2 so this was done as well:

    sgit -e 's/^### \(Original code\)/## \1/g' '*README.md'

2013/birken/README.md

@@ -51,7 +51,7 @@ If you wish to speed it up 200% you can use instead `-DZ=3750`. Doing this
 you might find the right value to your liking; use ctrl-c to terminate the
 program early.
 
-### Original code:
+## Original code:
 
 Should you wish to see the original without having to mess with the compiler
 line, try: