DOC: fix typo
DOC: plot_haxby_simple does face vs cats (and not houses)
DOC: display_mask is not used to plot the mask in plot_mask_computation.py
DOC: fix typo
DOC: fix link target and end-before target in plot_haxby_anova_svm.py
DOC: only mention NiftiMapsMasker since the content is under the "NiftiMapsMasker usage section"