Update migration.md

Author: fwang

www/docs/constructs/migration.md

@@ -65,7 +65,7 @@ Estimated time: 15 minutes
 
 Prerequisite: Update SST to [v0.59.0](https://github.com/serverless-stack/serverless-stack/releases/tag/v0.59.0) or later
 
-1. Run `npx sst update 1.0.0-beta.9`
+1. Run `npx sst update 1.0.0-beta.20`
 2. For each SST construct used in your app, find its corresponding section in the [Changelog](#changelog) below, and follow the steps to update.
 3. Ensure that all the constructs have been updated:
     - For TS projects (ie. using `index.ts`), run `npx run tsc --noEmit` or `yarn run tsc --noEmit` to ensure there are no type errors.