DOC: Fixed links to modules in the example gallery

examples/README.txt

examples/applications/README.txt

examples/cluster/README.txt

examples/covariance/README.txt

examples/decomposition/README.txt

examples/ensemble/README.txt

examples/gaussian_process/README.txt

examples/linear_model/README.txt

examples/manifold/README.txt

examples/mixture/README.txt

examples/neighbors/README.txt

examples/svm/README.txt

examples/tree/README.txt