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.

2005/klausler/README.md

@@ -4,7 +4,7 @@
     Cray Inc.
     2802 W 40th St
     Minneapolis MN 55410
-    USA
+    US
     pmk@cray.com
 
 ## To build:

2005/mikeash/README.md

@@ -1,7 +1,7 @@
 # Best use of parenthesis
 
     Michael Ash
-    USA
+    US
     mike@mikeash.com
 
 ## To build:

2005/vik/README.md

@@ -3,7 +3,7 @@
     Daniel Vik
     926 Genter Street
     La Jolla Ca 92037
-    USA
+    US
     daniel@vik.cc
 
 ## To build:

2005/vince/README.md

@@ -2,7 +2,7 @@
 
     Vincent Weaver
     Ithaca, NY
-    USA
+    US
     vince@deater.net
     <http://www.deater.net/weave/>