Fix typo in README

Author: blegat

README.md

@@ -5,7 +5,7 @@
 [![](https://zenodo.org/badge/DOI/10.5281/zenodo.1208672.svg)](https://doi.org/10.5281/zenodo.1208672)
 
 [SumOfSquares.jl](https://github.com/jump-dev/SumOfSquares.jl) is a JuMP
-extension that, when used in conjunction with [MultivariatePolynomial](https://github.com/JuliaAlgebra/MultivariatePolynomials.jl)
+extension that, when used in conjunction with [MultivariatePolynomials](https://github.com/JuliaAlgebra/MultivariatePolynomials.jl)
 and [PolyJuMP](https://github.com/jump-dev/PolyJuMP.jl), implements a sum of
 squares reformulation for polynomial optimization.