Add missing ':' to Alternate code sections

This was done as:

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

The heading levels and other things to do with alternate code that are
being discussed have not been addressed here. That will come later. This
is, as noted, just adding the missing ':' where it was missing.

The command does NOT touch it when it is not prefixed with '# ' because
there are cases where normal lines end with 'Alternate code' (these will
be turned into links later on).

2020/kurdyukov1/README.md

@@ -25,7 +25,7 @@ echo IOCCC | ./prog
 ./prog < prog.x86_64.asm
 ```
 
-### Alternate code
+### Alternate code:
 
 An even smaller alternate version of this entry, prog.alt.c, is provided.  This code does not contain any headers, nor any workaround for WIN32 based platforms.
 

2020/kurdyukov3/README.md

@@ -28,7 +28,7 @@ echo International Obfuscated C Code Contest | ./prog
 ./prog < input.txt
 ```
 
-### Alternate code
+### Alternate code:
 
 There is an alternate version of this code in `prog.alt.c` where the main code is a macro.