[DOCS] Fixed NDCG formula issues

When j is 0, log(j+1) will be 0, and this leads to division by 0 issue.

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## How was this patch tested?

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Please review http://spark.apache.org/contributing.html before opening a pull request.

Closes #22090 from yueguoguo/patch-1.

Authored-by: Zhang Le <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

docs/mllib-evaluation-metrics.md

@@ -462,13 +462,13 @@ $$rel_D(r) = \begin{cases}1 & \text{if $r \in D$}, \\ 0 & \text{otherwise}.\end{
       <td>Normalized Discounted Cumulative Gain</td>
       <td>
         $NDCG(k)=\frac{1}{M} \sum_{i=0}^{M-1} {\frac{1}{IDCG(D_i, k)}\sum_{j=0}^{n-1}
-          \frac{rel_{D_i}(R_i(j))}{\text{ln}(j+1)}} \\
+          \frac{rel_{D_i}(R_i(j))}{\text{ln}(j+2)}} \\
         \text{Where} \\
         \hspace{5 mm} n = \text{min}\left(\text{max}\left(|R_i|,|D_i|\right),k\right) \\
-        \hspace{5 mm} IDCG(D, k) = \sum_{j=0}^{\text{min}(\left|D\right|, k) - 1} \frac{1}{\text{ln}(j+1)}$
+        \hspace{5 mm} IDCG(D, k) = \sum_{j=0}^{\text{min}(\left|D\right|, k) - 1} \frac{1}{\text{ln}(j+2)}$
       </td>
       <td>
-        <a href="https://en.wikipedia.org/wiki/Information_retrieval#Discounted_cumulative_gain">NDCG at k</a> is a
+        <a href="https://en.wikipedia.org/wiki/Discounted_cumulative_gain#Normalized_DCG">NDCG at k</a> is a
         measure of how many of the first k recommended documents are in the set of true relevant documents averaged
         across all users. In contrast to precision at k, this metric takes into account the order of the recommendations
         (documents are assumed to be in order of decreasing relevance).