Fix compilation of 2018/burton2

I forgot about the DATA variable in the Makefile when changing the
discrepancies file to be a markdown file.

2018/burton2/Makefile

@@ -120,7 +120,7 @@ TARGET= ${PROG}
 ALT_OBJ=
 ALT_TARGET=
 
-EXTRA_DATA= ansi c++11 c++14 c++98 c11 c99 discrepancies \
+EXTRA_DATA= ansi c++11 c++14 c++98 c11 c99 discrepancies.md \
     test01.c test02.c test03.c test04.c test05.c test06.c test07.c \
     test08.c test09.c test10.c test11.c test12.c test13.c test14.c \
     iocccbug01.c iocccbug02.c iocccbug03.c iocccbug04.c iocccbug05.c \