Update index.md (#677)

The tools didn't have newlines between them making it hard to read

Author: wsmoses

docs/src/index.md

@@ -51,7 +51,9 @@ It also `unthunk`s every tangent.
 Several packages do not automatically load rules from ChainRules by default, but support importing rules that were defined using it, e.g. with a macro.
 
 [ReverseDiff.jl](https://github.com/JuliaDiff/ReverseDiff.jl) is a reverse-mode AD that supports using `rrule`s, but not calling back into AD and opting out of rules.
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 [Tracker.jl](https://github.com/FluxML/Tracker.jl) is a reverse mode AD that supports importing `rrules`
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 [Enzyme.jl](https://github.com/EnzymeAD/Enzyme.jl) is a forward and reverse mode AD and supports import of `frule`s and `rrule`s.
 
 [Tapir.jl](https://github.com/withbayes/Tapir.jl/) is a reverse-mode add that supports importing a restricted subset of rules defined using `rrule`. Specifically, rules for functions whose inputs have tangent type `Float64` or `NoTangent`.