Fix straight apostrophes in player-visible text.

Fix four instances — one in The Rise of Wesnoth’s dialog, two in the
ogre race description, and one in the “LAN server” “motd” — of the
straight apostrophe (U+0027 ' APOSTROPHE) being used in player-visible
text, where the typographic apostrophe (U+2019 ’ RIGHT SINGLE QUOTATION
MARK) ought to be used instead.