Update poseidon.md

Original reference: lurk-lab/neptune#113

The documentation says that w is the first column of m_hat or second column of m without the first row.. 

Actually it is the first column of m   without the first row.

The implementation is ok: https://github.com/filecoin-project/neptune/blob/bafd77a5014e3b6a6b40359097835c3eb1dd533f/src/mds.rs#L197

content/algorithms/crypto/poseidon.md

@@ -698,14 +698,10 @@ $\overline{\underline{\Function \textsf{sparse\_factorize}(m \typecolon \Zp^{[t
  0 & m_{t - 1, 1} & \dots & m_{t - 1, t - 1}
 \end{bmatrix} \\~
 \\
-\line{3} \wb \typecolon \Zp^{[t - 1 {\times} 1]} = \hat{m}_{\ast, 0} = \begin{bmatrix}
- \hat{m}_{0, 0} \\
+\line{3} \wb \typecolon \Zp^{[t - 1 {\times} 1]} = m_{1 \dotdot, 0} = \begin{bmatrix}
+ m_{1, 0} \\
  \vdots \\
- \hat{m}_{t - 2, 0}
-\end{bmatrix} = m_{1 \dotdot, 1} = \begin{bmatrix}
- m_{1, 1} \\
- \vdots \\
- m_{t - 1, 1}
+ m_{t - 1, 0}
 \end{bmatrix} \\~
 \\
 \line{4} \hat\wb \typecolon \Zp^{[t - 1 {\times} 1]} = \hat{m}^{\neg 1} {\times} \hso \wb = \begin{bmatrix}