[SPARK-21976][DOC] Fix wrong documentation for Mean Absolute Error.

FavioVazquez · srowen · commit e2ac2f1c71a0 · 2017-09-12T10:33:35.000+01:00
## What changes were proposed in this pull request? Fixed wrong documentation for Mean Absolute Error. Even though the code is correct for the MAE: ```scala Since("1.2.0") def meanAbsoluteError: Double = { summary.normL1(1) / summary.count } ``` In the documentation the division by N is missing. ## How was this patch tested? All of spark tests were run. Please review http://spark.apache.org/contributing.html before opening a pull request. Author: FavioVazquez <favio.vazquezp@gmail.com> Author: faviovazquez <favio.vazquezp@gmail.com> Author: Favio André Vázquez <favio.vazquezp@gmail.com> Closes #19190 from FavioVazquez/mae-fix.
diff --git a/docs/mllib-evaluation-metrics.md b/docs/mllib-evaluation-metrics.md
@@ -549,7 +549,7 @@ variable from a number of independent variables.
     </tr>
     <tr>
       <td>Mean Absolute Error (MAE)</td>
-      <td>$MAE=\sum_{i=0}^{N-1} \left|\mathbf{y}_i - \hat{\mathbf{y}}_i\right|$</td>
+      <td>$MAE=\frac{1}{N}\sum_{i=0}^{N-1} \left|\mathbf{y}_i - \hat{\mathbf{y}}_i\right|$</td>
     </tr>
     <tr>
       <td>Coefficient of Determination $(R^2)$</td>
