fix: added default values for variables as per readme

Author: pawan1210

examples/internal-lb-cloud-run/variables.tf

@@ -20,18 +20,22 @@ variable "project_id" {
 
 variable "backend_region_a" {
   type = string
+  default = "us-east1"
 }
 
 variable "backend_region_b" {
   type = string
+  default = "us-south1"
 }
 
 variable "subnet_region_a" {
   type = string
+  default = "us-central1"
 }
 
 variable "subnet_region_b" {
   type = string
+  default = "us-west1"
 }