Make board sizes consistent: Uses both "NUMBER × NUMBER" and "NUMBER×NUMBER"
Board sizes are written with "NUMBER × NUMBER" in Tetravex it self but with "NUMBER×NUMBER" in the help. Suggest making it the same.
https://gitlab.gnome.org/GNOME/gnome-tetravex/-/blob/master/help/C/size.page#L28
https://gitlab.gnome.org/GNOME/gnome-tetravex/-/blob/master/src/app-menu.ui#L32-56 https://gitlab.gnome.org/GNOME/gnome-tetravex/-/blob/master/src/gnome-tetravex.vala#L776 https://gitlab.gnome.org/GNOME/gnome-tetravex/-/blob/master/src/gnome-tetravex.vala#L808 https://gitlab.gnome.org/GNOME/gnome-tetravex/-/blob/master/src/score-dialog.vala#L144-145