Changing heading level of 'Alternate code'

Change level 3 (###) to level 2 (##):

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

which is the same as:

    sgit -e 's/^### Alternate code/## Alternate code/g' '*README.md'

This is per discussion in GitHub.

1986/marshall/README.md

@@ -23,7 +23,7 @@ to read it for entertainment if nothing else.
 ./marshall
 ```
 
-### Alternate code:
+## Alternate code:
 
 As some changes had to be made due to the problem of conflicting compiler
 options briefly noted above we provide the code that demonstrates the problem so