DOC spellfixes

doc/datasets/labeled_faces.rst

doc/datasets/twenty_newsgroups.rst

doc/developers/index.rst

doc/developers/performance.rst

doc/modules/clustering.rst

doc/modules/cross_validation.rst

doc/modules/decomposition.rst

doc/modules/dp-derivation.rst

doc/modules/ensemble.rst

doc/modules/feature_extraction.rst

doc/modules/feature_selection.rst

doc/modules/gaussian_process.rst

doc/modules/grid_search.rst

doc/modules/hmm.rst

doc/modules/kernel_approximation.rst

doc/modules/linear_model.rst

doc/modules/manifold.rst

doc/modules/model_evaluation.rst

doc/modules/multiclass.rst

doc/modules/outlier_detection.rst