docs: TOC fix in upgrade-to-v5.md

Author: dariakp

docs/upgrade-to-v5.md

@@ -7,6 +7,7 @@
     - [Impacted APIs now return `Uint8Array` in web environments; Node.js environments are unaffected](#apis-impacted)
   - [Restrict supported encodings in `ObjectId.toString` / `UUID.toString` / `Binary.toString`](#restrict-supported-encodings-in-objectidtostring--uuidtostring--binarytostring)
     - [Migration available if types beyond `'hex' | 'base64' | 'utf8'` are desired](#migration-example)
+  - [Bundling Top-Level Await in BSON](#bundling-top-level-await-in-bson)
 - [Other Changes](#other-changes)
   - [`serializeFunctions` bug fix](#serializefunctions-bug-fix)
   - [TS "target" set to es2020](#ts-target-set-to-es2020)