Documentation fix: Various spelling mistakes (and one use of the wrong verb) across...

Documentation fix: Various spelling mistakes (and one use of the wrong verb) across a few domains that show up in documentation.

| domain | current | suggestion |
|--------|---------|------------|
| [GtkButton](https://gitlab.gnome.org/GNOME/gtk/-/blob/main/gtk/gtkbutton.c#L691)       | "Buttons can has a flat appearance" | "Buttons can have a flat appearance" |
| [GtkCenterBox](https://gitlab.gnome.org/GNOME/gtk/-/blob/main/gtk/gtkcenterbox.c#L275) | "pas %NULL" | "pass %NULL" |
| [GtkEditable](https://gitlab.gnome.org/GNOME/gtk/-/blob/main/gtk/gtkeditable.c#L466)   | "the text to append" | "the text to insert" |
| [GtkFlowbox](https://gitlab.gnome.org/GNOME/gtk/-/blob/main/gtk/gtkflowbox.c#L4921)    | "the are equal" | "they are equal" |
| [GtkSelectionModel](https://gitlab.gnome.org/GNOME/gtk/-/blob/main/gtk/gtkselectionmodel.c#L550) | "Call this when a the selection changes" | "Call this when a selection changes" |
| [GtkWIndow](https://gitlab.gnome.org/GNOME/gtk/-/blob/main/gtk/gtkwindow.c#L5321)                | "(e.g. the user or the window manager" | "(e.g. the user or the window manager)" |