Improve the notation for exponents such as 2e-15

[OscarcarLi]

Thanks for the great resource on mixed-precision training! In the section https://docs.fast.ai/callback.fp16.html#problems-with-half-precision, the notation such as 2e-15 is used. This notation commonly means $2 \times 10^{-15}$, but I believe what was meant is $2^{-15}$. I think choosing a different notation will prevent potential confusion. Thanks!