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.

1991/buzzard/README.md

@@ -4,7 +4,7 @@ Sean Barrett
 University of Maryland  
 5407 20th Place   
 Hyattsville MD 20782   
-USA  
+US  
 
 ## To build:
 

1991/davidguy/README.md

@@ -4,14 +4,14 @@ David Applegate
 School of Computer Science	
 Carnegie Mellon University	
 Pittsburgh PA 15213		
-USA				
+US				
 
 
 Guy Jacobson  
 AT&T Bell Laboratories  
 600 Mountain Avenue  
 Murray Hill NJ 07974  
-USA  
+US  
 
 
 ## To build:

1991/fine/README.md

@@ -5,7 +5,7 @@ Ohio State University
 Department of Computer and Information Science  
 2036 Neil Avenue Mall  
 Columbus, Ohio  43210  
-USA  
+US  
 
 ## To build:
 

1991/westley/README.md

@@ -4,7 +4,7 @@ Brian Westley (Merlyn LeRoy on usenet)
 DigiBoard, Inc.  
 1026 Blair Ave.  
 St. Paul, MN  55104  
-USA   
+US   
 <http://www.westley.org>  
 
 ## To build: