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.

1988/robison/README.md

@@ -45,7 +45,7 @@ This program shows that C has many unnecessary constructs, in fact
 only `while`,`--`, and `>=` are required.  (The two assignments at
 the beginning could be avoided if `atoi(3)` was rewritten with this
 new paradigm.)  Note that the lack of both the controversial `goto`
-and assignment statements makes the meaning crystal clear.  The current 
+and assignment statements makes the meaning crystal clear.  The current\
 ANSI committee should look into this practical simplification of C.
 
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