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.

2020/burton/README.md

@@ -37,7 +37,7 @@ arg. It might also print strange output with more than one arg.
 ./prog 128 128 128 128
 ```
 
-### Alternate code:
+## Alternate code:
 
 By default, this code compiles for Little Endian machines.
 An alternate version is also compiled for Big Endian machines.