Update README.md

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README.md

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-GreenFunc.jl is a differentiable numerical framework to manipulate multidimensional Green's functions.
+GreenFunc.jl is a numerical framework to manipulate multidimensional Green's functions.
 
 ## Features
  - `MeshArray` type as an array defined on meshes, which provides a generic data structure for Green's functions, vertex functions or any other correlation/response functions.