Remove 'America' (A) from 'USA' :-)

This to make the country code consistent with mkiocccentry (and I guess
international standard country codes).

This affects many README.md files and (for one line) winners.csv.

1995/schnitzi/README.md

@@ -4,7 +4,7 @@ Mark Schnitzius
 ISX Corporation  
 3215-C Post Woods Dr.  
 Atlanta, GA 30339   
-USA  
+US  
 <http://computronium.org/ioccc.html>