Update README jacobian_sparsity

[bowenszhu]

close #193

• Remove SparsityDetection.jl from the text description.
• Symbolics.jacobian_sparsity returns a SparseMatrixCSC. One doesn't need to call sparse.
  https://github.com/JuliaSymbolics/Symbolics.jl/blob/e45badffda9bf5e4ba8e76914c171a73a08346c1/src/diff.jl#L527
  https://github.com/JuliaSparse/SparseArrays.jl/blob/1bae96dc8f9a8ca8b7879eef4cf71e186598e982/src/sparsematrix.jl#L960

[codecov-commenter]

Codecov Report

Base: 81.40% // Head: 81.40% // No change to project coverage 👍

Coverage data is based on head (8ff4930) compared to base (e89cc31).
Patch has no changes to coverable lines.

Additional details and impacted files

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  Coverage   81.40%   81.40%           
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  Files          15       15           
  Lines         898      898           
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  Hits          731      731           
  Misses        167      167           

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