Revert "README.dm: Update timely-dataflow link to the current repository." #710

Job	Run time
test (7.7.0, 3.7)	2m 41s
check-minimal-versions	1m 7s
test (7.5.1, 3.6)	2m 40s
lint	2m 21s
check (macos-11.0)	1s
test (7.5.1, 3.5)	2m 33s
check (windows-2019, cmake-build,libz-static,curl-static, cmake-build,libz-static,curl-static)	5m 11s
check (ubuntu-20.04, tracing)	1m 58s
check (ubuntu-20.04, cmake-build,ssl-vendored,gssapi-vendored,libz-static,curl-static,zstd, cmake...	4m 54s
	23m 26s

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