Fix warning indentation (#250)

Author: gdalle

DifferentiationInterface/docs/src/backends.md

@@ -71,11 +71,11 @@ backend_table #hide
 !!! danger "Compatibility with Julia 1.6"
     As of version 0.3.4, DifferentiationInterface.jl is compatible with Julia 1.6, the Long Term Support (LTS) version of the language.
     However, we were only able to test the following backends on LTS:
-        - FiniteDifferences.jl
-        - ForwardDiff.jl
-        - ReverseDiff.jl
-        - Tracker.jl
-        - Zygote.jl
+      - FiniteDifferences.jl
+      - ForwardDiff.jl
+      - ReverseDiff.jl
+      - Tracker.jl
+      - Zygote.jl
     We strongly recommend that users upgrade to Julia 1.10, where all backends are tested.
 
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