Update Imperialism 1974 branch. (#10466)

Author: asvitkine

triplea_maps.yaml

@@ -2632,8 +2632,8 @@
    <br>Based upon the Axis & Allies: 1914 boardgame
 - mapName: Imperialism 1974
   mapCategory: EXPERIMENTAL
-  url: https://github.com/triplea-maps/imperialism_1974/archive/master.zip
+  url: https://github.com/triplea-maps/imperialism_1974_board_game/archive/master.zip
   version: 1
-  img: https://raw.githubusercontent.com/triplea-maps/imperialism_1974/master/preview.png
+  img: https://raw.githubusercontent.com/triplea-maps/imperialism_1974_board_game/master/preview.png
   description: |
    <br>Adaptation of the 1974 board game "Imperialism"