DOC add link to fetch_covertype to covertype narrative docs

doc/datasets/covtype.rst

@@ -13,7 +13,7 @@ Each sample has 54 features, described on the
 Some of the features are boolean indicators,
 while others are discrete or continuous measurements.
 
-``sklearn.datasets.fetch_covtype`` will load the covertype dataset;
+:func:`sklearn.datasets.fetch_covtype` will load the covertype dataset;
 it returns a dictionary-like object
 with the feature matrix in the ``data`` member
 and the target values in ``target``.