minor polishing (#383)
minor polishing (#383)

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

* explain the meaning of x in chapter optimization gd-sgd.md

* minor polishing

* minor polishing

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