StyleManager: Fix build error with valadoc

[ryonakano]

Fixed the following errors by valadoc when you build Granite with -Ddocumentation=true

StyleManager.vala:47.131-47.132: error: expected }: <space>
     * Uses value from {@link Granite.Settings.prefers_color_scheme} when set to {@link Granite.Settings.ColorScheme.NO_PREFERENCE }.
                                                                                                                                  ^   
StyleManager.vala:24.51-24.52: error: expected }: <space>
     * as gotten by {@link Gdk.Display.get_default ()}.
                                                  ^     
Failed: 2 error(s), 10 warning(s)

[danirabbit]

I guess we should have the CI set with those flags?