Update anchor.md (d2l-ai#804)

We need to traverse $A_1, A_3, A_4, A_6, A_8$ as we have already eliminated $A_2, A_5, A_7, A_9$

chapter_computer-vision/anchor.md

@@ -113,7 +113,7 @@ We proceed until all elements in the $n_b$ column in the matrix $\mathbf{X}$ are
 Next, we only traverse the remaining $n_a - n_b$ anchor boxes. Given anchor box $A_i$, find the bounding box $B_j$ with the largest IoU with $A_i$ according to the $i^\mathrm{th}$ row of the matrix $\mathbf{X}$, and only assign ground-truth bounding box $B_j$ to anchor box $A_i$ when the IoU is greater than the predetermined threshold.
 
 
-As shown in :numref:`fig_anchor_label` (left), assuming that the maximum value in the matrix $\mathbf{X}$ is $x_{23}$, we will assign ground-truth bounding box $B_3$ to anchor box $A_2$. Then, we discard all the elements in row 2 and column 3 of the matrix, find the largest element $x_{71}$ of the remaining shaded area, and assign ground-truth bounding box $B_1$ to anchor box $A_7$. Then, as shown in :numref:`fig_anchor_label`  (middle), discard all the elements in row 7 and column 1 of the matrix, find the largest element $x_{54}$ of the remaining shaded area, and assign ground-truth bounding box $B_4$ to anchor box $A_5$.  Finally, as shown in :numref:`fig_anchor_label` (right), discard all the elements in row 5 and column 4 of the matrix, find the largest element $x_{92}$ of the remaining shaded area, and assign ground-truth bounding box $B_2$ to anchor box $A_9$.  After that, we only need to traverse the remaining anchor boxes of $A_2, A_5, A_7, A_9$ and determine whether to assign ground-truth bounding boxes to the remaining anchor boxes according to the threshold.
+As shown in :numref:`fig_anchor_label` (left), assuming that the maximum value in the matrix $\mathbf{X}$ is $x_{23}$, we will assign ground-truth bounding box $B_3$ to anchor box $A_2$. Then, we discard all the elements in row 2 and column 3 of the matrix, find the largest element $x_{71}$ of the remaining shaded area, and assign ground-truth bounding box $B_1$ to anchor box $A_7$. Then, as shown in :numref:`fig_anchor_label`  (middle), discard all the elements in row 7 and column 1 of the matrix, find the largest element $x_{54}$ of the remaining shaded area, and assign ground-truth bounding box $B_4$ to anchor box $A_5$.  Finally, as shown in :numref:`fig_anchor_label` (right), discard all the elements in row 5 and column 4 of the matrix, find the largest element $x_{92}$ of the remaining shaded area, and assign ground-truth bounding box $B_2$ to anchor box $A_9$.  After that, we only need to traverse the remaining anchor boxes of $A_1, A_3, A_4, A_6, A_8$ and determine whether to assign ground-truth bounding boxes to the remaining anchor boxes according to the threshold.
 
 ![Assign ground-truth bounding boxes to anchor boxes. ](../img/anchor-label.svg)
 :label:`fig_anchor_label`