update equation of adam
update equation of adam

epsilon should be outside of sqrt instead of inside (the code is right, the equation has the typo)
d2l-ai/d2l-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh,d2l-ai/d2l-zh,d2l-ai/d2l-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh