Merge pull request #714 from maarten-keijzer/doc_fix

Updated the documentation for 'gradient' and 'Hessian' (subscript error)

doc/model.md

@@ -150,8 +150,8 @@ Obj^{(t)} = \sum_{i=1}^n [l(y_i, \hat{y}_i^{(t-1)}) + g_i f_t(x_i) + \frac{1}{2}
 where the ``$g_i$`` and ``$h_i$`` are defined as
 
 ```math
-g_i &= \partial_{\hat{y}_i^{(t)}} l(y_i, \hat{y}_i^{(t-1)})\\
-h_i &= \partial_{\hat{y}_i^{(t)}}^2 l(y_i, \hat{y}_i^{(t-1)})
+g_i &= \partial_{\hat{y}_i^{(t-1)}} l(y_i, \hat{y}_i^{(t-1)})\\
+h_i &= \partial_{\hat{y}_i^{(t-1)}}^2 l(y_i, \hat{y}_i^{(t-1)})
 ```
 
 After we remove all the constants, the specific objective at step ``$t$`` becomes