metrics: Rename C-Ray to cpu performance tests

This PR renames C-Ray tests to cpu category.

Signed-off-by: Gabriela Cervantes <gabriela.cervantes.tellez@intel.com>

tests/metrics/README.md

@@ -76,8 +76,6 @@ For further details see the [storage tests documentation](storage).
 
 Test relating to measure reading and writing against clusters.
 
-For further details see the [disk tests documentation](disk).
-
 ### Machine Learning
 
 Tests relating with TensorFlow and Pytorch implementations of several popular
@@ -85,6 +83,12 @@ convolutional models.
 
 For further details see the [machine learning tests documentation](machine_learning).
 
+### `CPU`
+
+Tests related with `CPU` performance.
+
+For further details see the [`cpu` tests documentation](cpu).
+
 ## Saving Results
 
 In order to ensure continuity, and thus testing and historical tracking of results,