dj.1, strcmp.1, swab.1: remove nonexistent roff macros

docs/strcmp.1

@@ -25,14 +25,14 @@ will exit with an error code of 1 if a string passed has a lesser byte value
 than one of the prior strings:
 
 .RS
-.R strcmp b a
+strcmp b a
 .RE
 
 and with an error code of 255 if it has a greater byte value than one of the
 prior strings:
 
 .RS
-.R strcmp a b
+strcmp a b
 .RE
 
 When invoked incorrectly, a debug message will be printed and the program will