doc: fix comm's --help output so we generate a better man page

* src/comm.c (usage): Don't align example comments in --help output,
since the extra space (sequence of two spaces) there would be
interpreted by help2man and induce an unwanted line break
in the resulting man page.  Reported by Jari Aalto.

src/comm.c

@@ -139,7 +139,7 @@ Note, comparisons honor the rules specified by `LC_COLLATE'.\n\
 \n\
 Examples:\n\
   %s -12 file1 file2  Print only lines present in both file1 and file2.\n\
-  %s -3  file1 file2  Print lines in file1 not in file2, and vice versa.\n\
+  %s -3 file1 file2  Print lines in file1 not in file2, and vice versa.\n\
 "),
               program_name, program_name);
       emit_ancillary_info ();