Edit defaultmap for active duty map change

Dust2 has been taken out of active duty. Changed map to mirage and changed mappool to mg_active following the Valve documentation: 
https://developer.valvesoftware.com/wiki/Counter-Strike:_Global_Offensive_Dedicated_Servers#Starting_the_Server

CounterStrikeGlobalOffensive/csgoserver

@@ -32,8 +32,8 @@ version="170128"
 # Deathmatch				1			2
 gametype="0"
 gamemode="0"
-defaultmap="de_dust2"
-mapgroup="random_classic"
+defaultmap="de_mirage"
+mapgroup="mg_active"
 maxplayers="16"
 tickrate="64"
 port="27015"
@@ -220,4 +220,4 @@ fi
 core_dl.sh
 core_functions.sh
 getopt=$1
-core_getopt.sh
+core_getopt.sh