fix(ec2): r5ad instance-type has incorrect value (aws#14179)

The InstanceClass enum has incorrectly mapped values for R5AD instances.

I am not sure whether this constitutes a breaking change, given that the value is incorrect and would likely not work for any users attempting to launch an R5AD instance. 

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-ec2/lib/instance-types.ts

@@ -141,12 +141,12 @@ export enum InstanceClass {
   /**
    * Memory optimized instances based on AMD EPYC with local NVME drive, 5th generation
    */
-  MEMORY5_AMD_NVME_DRIVE = 'r5a',
+  MEMORY5_AMD_NVME_DRIVE = 'r5ad',
 
   /**
    * Memory optimized instances based on AMD EPYC with local NVME drive, 5th generation
    */
-  R5AD = 'r5a',
+  R5AD = 'r5ad',
 
   /**
    * Memory optimized instances that are also EBS-optimized, 5th generation

packages/@aws-cdk/aws-ec2/test/instance.test.ts

@@ -138,6 +138,39 @@ nodeunitShim({
 
     test.done();
   },
+  'instances with local NVME drive are correctly named'(test: Test) {
+    // GIVEN
+    const sampleInstanceClassKeys = [{
+      key: 'R5D',
+      value: 'r5d',
+    }, {
+      key: 'MEMORY5_NVME_DRIVE',
+      value: 'r5d',
+    }, {
+      key: 'R5AD',
+      value: 'r5ad',
+    }, {
+      key: 'MEMORY5_AMD_NVME_DRIVE',
+      value: 'r5ad',
+    }, {
+      key: 'M5AD',
+      value: 'm5ad',
+    }, {
+      key: 'STANDARD5_AMD_NVME_DRIVE',
+      value: 'm5ad',
+    }]; // A sample of instances with NVME drives
+
+    for (const instanceClass of sampleInstanceClassKeys) {
+      // WHEN
+      const key = instanceClass.key as keyof(typeof InstanceClass);
+      const instanceType = InstanceClass[key];
+
+      // THEN
+      expect(instanceType).toBe(instanceClass.value);
+    }
+
+    test.done();
+  },
   'instance architecture throws an error when instance type is invalid'(test: Test) {
     // GIVEN
     const malformedInstanceTypes = ['t4', 't4g.nano.', 't4gnano', ''];