Add a link to the metrics document (keras-team#13334)

Link to the metrics document(/metrics) was missing in 'Compilation' section. Added one just as other explained arguments.

Author: Junyoung Kim

docs/templates/getting-started/sequential-model-guide.md

@@ -52,7 +52,7 @@ Before training a model, you need to configure the learning process, which is do
 
 - An optimizer. This could be the string identifier of an existing optimizer (such as `rmsprop` or `adagrad`), or an instance of the `Optimizer` class. See: [optimizers](/optimizers).
 - A loss function. This is the objective that the model will try to minimize. It can be the string identifier of an existing loss function (such as `categorical_crossentropy` or `mse`), or it can be an objective function. See: [losses](/losses).
-- A list of metrics. For any classification problem you will want to set this to `metrics=['accuracy']`. A metric could be the string identifier of an existing metric or a custom metric function.
+- A list of metrics. For any classification problem you will want to set this to `metrics=['accuracy']`. A metric could be the string identifier of an existing metric or a custom metric function. See: [metrics](/metrics).
 
 ```python
 # For a multi-class classification problem