CLN: Capitalize "Gaussian" in example docstrings

examples/cluster/plot_kmeans_stability_low_dim_dense.py

examples/covariance/plot_lw_vs_oas.py

examples/covariance/plot_mahalanobis_distances.py

examples/decomposition/plot_image_denoising.py

examples/decomposition/plot_sparse_coding.py

examples/linear_model/plot_ard.py

examples/linear_model/plot_bayesian_ridge.py

examples/linear_model/plot_logistic.py

examples/mixture/plot_gmm.py

examples/mixture/plot_gmm_pdf.py

examples/mixture/plot_gmm_selection.py

examples/mixture/plot_gmm_sin.py

examples/neighbors/plot_kde_1d.py

examples/plot_lda_qda.py