replace trailing whitespace with blackslash in README.md

As an alternative to the markdown trailing whitespace as a line
break, we use a trailing blackslash as a markdown line break.

1990/dg/README.md

@@ -1,9 +1,9 @@
 # Best Abuse of the C Preprocessor
 
-David Goodenough  
-anonymous organization   
-541 Commonwealth Ave  
-Newton, MA 02159  
+David Goodenough\
+anonymous organization\
+541 Commonwealth Ave\
+Newton, MA 02159\
 US
 
 ## To build: