Mistake in solution to putnam_like_set3_b4

putnam_like_set3_b4. There is a mistake that does not impact the answer and the idea of this solution. Last step in grading_scheme.md should be as follows.

\bs[3]
Let $N$ be an integer in $[N_n,N_{n+1}]$ i.e. $N=N_n+m$. Then
\[
b_N=\frac{b_{N_n}\cdot N_n^\alpha +1+2+\ldots+m}{(N_n+m)^{\alpha}}
\]
and we estimate
\[
b_{N_n}\cdot\frac{N_n^{\alpha}}{N_{n+1}^{\alpha}}