docs(secretsmanager): remove reference to Secret.grantUpdate (#19151)

This method does not exist.  This mention in the docs is leftover from an abandoned approach in #8600 and should have been reverted.


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Author: isker

packages/@aws-cdk/aws-secretsmanager/README.md

@@ -63,7 +63,7 @@ A secret can set `RemovalPolicy`. If it set to `RETAIN`, that removing a secret
 ## Grant permission to use the secret to a role
 
 You must grant permission to a resource for that resource to be allowed to
-use a secret. This can be achieved with the `Secret.grantRead` and/or `Secret.grantUpdate`
+use a secret. This can be achieved with the `Secret.grantRead` and/or `Secret.grantWrite`
  method, depending on your need:
 
 ```ts