BUG/MINOR: makefile: fix a tiny typo in the target list

Previous commit 382001b46 ("BUILD: Add a DragonFlyBSD target") introduced
a tiny typo in the target list ("iopenbs" vs "openbsd"). This will have to
be backported if that patch is backported.
diff --git a/Makefile b/Makefile
index 4aabe34..ecb034d 100644
--- a/Makefile
+++ b/Makefile
@@ -782,7 +782,7 @@
 	@echo "Please choose the target among the following supported list :"
 	@echo
 	@echo "   linux-glibc, linux-glibc-legacy, linux-musl, solaris, freebsd, dragonfly, "
-	@echo "   iopenbs, netbsd, cygwin, haiku, aix51, aix52, aix72-gcc, osx, generic, "
+	@echo "   openbsd, netbsd, cygwin, haiku, aix51, aix52, aix72-gcc, osx, generic, "
 	@echo "   custom"
 	@echo
 	@echo "Use \"generic\" if you don't want any optimization, \"custom\" if you"