fix function f(x)=x_1^2+2x_2^2 in chapter_optimization gd-sgd.md (#374)
fix function f(x)=x_1^2+2x_2^2 in chapter_optimization gd-sgd.md (#374)


d2l-ai/d2l-zh,mli/gluon-tutorials-zh,d2l-ai/d2l-zh,d2l-ai/d2l-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh