syntax: add forgotten `https` to `cabal.syntax` #253

Job	Run time
call-build-macos / build-macos	2m 5s
call-build-fedora / build-fedora	1m 54s
call-build-alpine / build-alpine	2m 3s
call-build-freebsd / build-freebsd	3m 14s
call-build-solaris / build-solaris	6m 25s
call-build-ubuntu / build-ubuntu	4m 22s
	20m 3s

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