Update paired region

Author: Zubair Abid

src/azure-cli/azure/cli/command_modules/backup/custom.py

@@ -92,6 +92,7 @@
     "southafricawest": "southafricanorth",
     "southcentralus": "northcentralus",
     "southeastasia": "eastasia",
+    "southeastus": "westus3",
     "southindia": "centralindia",
     "swedencentral": "swedensouth",
     "swedensouth": "swedencentral",