README: update codecov badge

Author: inkydragon

README.md

@@ -3,7 +3,7 @@
 A general framework for fast Fourier transforms (FFTs) in Julia.
 
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-[![Codecov](http://codecov.io/github/JuliaMath/AbstractFFTs.jl/coverage.svg?branch=master)](http://codecov.io/github/JuliaMath/AbstractFFTs.jl?branch=master)
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