fix(lambda): Update RuntimeManagementConfig enum to be correct

According to the Lambda docs, there is no space for FunctionUpdate.
This commit uses the correct value, as specified here:

https://docs.aws.amazon.com/lambda/latest/dg/API_PutRuntimeManagementConfig.html#API_PutRuntimeManagementConfig_RequestSyntax

packages/@aws-cdk/aws-lambda/lib/runtime-management.ts

@@ -16,7 +16,7 @@ export class RuntimeManagementMode {
    * With this mode, you can detect and mitigate rare runtime update incompatibilities early.
    * When using this mode, you must regularly update your functions to keep their runtime up to date.
    */
-  public static readonly FUNCTION_UPDATE = new RuntimeManagementMode('Function update');
+  public static readonly FUNCTION_UPDATE = new RuntimeManagementMode('FunctionUpdate');
   /**
    * You specify a runtime version in your function configuration.
    * The function uses this runtime version indefinitely.

packages/@aws-cdk/aws-lambda/test/runtime-management.test.ts

@@ -20,7 +20,7 @@ describe('runtime', () => {
       },
     });
   });
-  test('Runtime Management Function Update', () => {
+  test('Runtime Management FunctionUpdate', () => {
     // GIVEN
     const app = new cdk.App();
     const stack = new cdk.Stack(app, 'stack');
@@ -33,7 +33,7 @@ describe('runtime', () => {
     // WHEN
     Template.fromStack(stack).hasResourceProperties('AWS::Lambda::Function', {
       RuntimeManagementConfig: {
-        UpdateRuntimeOn: 'Function update',
+        UpdateRuntimeOn: 'FunctionUpdate',
       },
     });
   });