correct links to backends in main module description #500

Job	Run time
cargo-doc	29s
msrv	12s
test_all_features	41s
build_and_test (ubuntu-latest)	34s
run_all_examples	18s
build_and_test (macos-latest)	44s
build_and_test (windows-latest)	47s
	3m 45s

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