Update README.md

Author: jakwisn

README.md

@@ -131,7 +131,7 @@ Where `i` denotes the membership to unique subgroup from protected variable. Unp
 some fairness metrics like *Equalized odds* are satisfied if parity loss in both *TPR* and *FPR* is low 
 
 #### How easy it is to add custom fairness metric? 
-It is relatively easy! [Check it out here](https://github.com/ModelOriented/fairmodels/blob/master/man/instructions/adding_new_metrics.png) 
+It is relatively easy! [Check it out here](https://github.com/ModelOriented/fairmodels/blob/master/man/instructions/adding_new_metric.md) 
 
 ### Fairness in regression
 R package fairmodels has support for regression models. Check fairness using `fairness_check_regression()` to approximate classification fairness metrics in regression setting. Plot object with `plot()` to visualize *fairness check* or with `plot_density()` to see model's output.