AchievementManager: Make GetInstance() return a reference

The internal static member will always have a valid lifetime. Makes this
consistent with other instance based objects in our code.

Source/Core/Core/Boot/Boot.cpp

@@ -580,8 +580,8 @@ bool CBoot::BootUp(Core::System& system, const Core::CPUThreadGuard& guard,
       }
 
 #ifdef USE_RETRO_ACHIEVEMENTS
-      AchievementManager::GetInstance()->HashGame(executable.path,
-                                                  [](AchievementManager::ResponseType r_type) {});
+      AchievementManager::GetInstance().HashGame(executable.path,
+                                                 [](AchievementManager::ResponseType r_type) {});
 #endif  // USE_RETRO_ACHIEVEMENTS
 
       if (!executable.reader->LoadIntoMemory(system))