Update custom-conditions.mdx

Made a change to code example within the *Preconditions and Postconditions* section so that it technically makes sense; prior it was missing the data resource that was being called within the precondition lifecycle event on line 135, and the aws_instance resource was not utilizing the ami being provided by the data source in line 129, so i changed that as well.

website/docs/language/expressions/custom-conditions.mdx

@@ -115,9 +115,18 @@ The following example shows use cases for preconditions and postconditions. The
 
 ```hcl
 
+data "aws_ami" "example" {
+  owners = ["amazon"]
+
+  filter {
+    name   = "image-id"
+    values = ["ami-abc123"]
+  }
+}
+
 resource "aws_instance" "example" {
-  instance_type = "t2.micro"
-  ami           = "ami-abc123"
+  instance_type = "t3.micro"
+  ami           = data.aws_ami.example.id
 
   lifecycle {
     # The AMI ID must refer to an AMI that contains an operating system