docs: Update README Usage section to reflect array format for identity_sources (#111)

Author: bigdragon0610

README.md

@@ -59,7 +59,7 @@ module "api_gateway" {
   authorizers = {
     "azure" = {
       authorizer_type  = "JWT"
-      identity_sources = "$request.header.Authorization"
+      identity_sources = ["$request.header.Authorization"]
       name             = "azure-auth"
       jwt_configuration = {
         audience         = ["d6a38afd-45d6-4874-d1aa-3c5c558aqcc2"]