build: Rename md5 to checksums as part of .PHONY target

In commit 46d03d37bf I renamed a Makefile target
from md5 to checksums, (as we switched from MD5 checksums to SHA-256
checksums, so the more general name is more future proof).

But that commit missed one mention of "md5" as a dependency of the .PHONY
target. Rename that here as well.

Makefile.am

@@ -102,4 +102,4 @@ checksums: $(ARCHIVES)
 	@-sha256sum $(PACKAGE_NAME).tar.bz2
 	@-sha256sum $(PACKAGE_NAME).zip
 
-.PHONY: tarballs md5
+.PHONY: tarballs checksums