Linear regression Tutorial link (apache#6468)

* Fixing a link in the linear regression tutorial.

The link was initally going to mxnet-test.readthedocs.io. Changed it to mxnet.io/api.

* More appropriate language.

docs/tutorials/python/linear-regression.md

@@ -171,7 +171,7 @@ eval_iter = mx.io.NDArrayIter(eval_data, eval_label, batch_size, shuffle=False)
 model.score(eval_iter, metric)
 ```
 
-We also can create a custom metric and use it to evaluate the model. More
-information on metrics can be found [here](http://mxnet-test.readthedocs.io/en/latest/api/metric.html).
+We can also create a custom metric and use it to evaluate a model. More
+information on metrics can be found in the [API documentation](http://mxnet.io/api/python/model.html#evaluation-metric-api-reference).
 
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