docs: adds verify ID Token IAP to samples table of contents

This PR was generated using Autosynth. 🌈

Commits in this repo since last synth:
c532b14 chore(deps): update dependency linkinator to v2 (#886)
a98e386 feat: support for verifying ES256 and retrieving IAP public keys (#887)
e671629 build: add GitHub actions config for unit tests (#888)
163e43d test: modernize mocha config (#884)
58e5029 chore: skip img.shields.io in docs test
13e8cfb chore(deps): update dependency @types/mocha to v7
634cf6a chore: release 5.9.2 (#882)
63c4637 fix: populate credentials.refresh_token if provided (#881)


<details><summary>Log from Synthtool</summary>

```
synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
On branch autosynth
nothing to commit, working tree clean
HEAD detached at FETCH_HEAD
nothing to commit, working tree clean
.eslintignore
.eslintrc.yml
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.github/PULL_REQUEST_TEMPLATE.md
.github/release-please.yml
.github/workflows/ci.yaml
.jsdoc.js
.kokoro/common.cfg
.kokoro/continuous/node10/common.cfg
.kokoro/continuous/node10/docs.cfg
.kokoro/continuous/node10/lint.cfg
.kokoro/continuous/node10/samples-test.cfg
.kokoro/continuous/node10/system-test.cfg
.kokoro/continuous/node10/test.cfg
.kokoro/continuous/node12/common.cfg
.kokoro/continuous/node12/test.cfg
.kokoro/continuous/node8/common.cfg
.kokoro/continuous/node8/test.cfg
.kokoro/docs.sh
.kokoro/lint.sh
.kokoro/presubmit/node10/common.cfg
.kokoro/presubmit/node10/docs.cfg
.kokoro/presubmit/node10/lint.cfg
.kokoro/presubmit/node10/samples-test.cfg
.kokoro/presubmit/node10/system-test.cfg
.kokoro/presubmit/node10/test.cfg
.kokoro/presubmit/node12/common.cfg
.kokoro/presubmit/node12/test.cfg
.kokoro/presubmit/node8/common.cfg
.kokoro/presubmit/node8/test.cfg
.kokoro/presubmit/windows/common.cfg
.kokoro/presubmit/windows/test.cfg
.kokoro/publish.sh
.kokoro/release/docs.cfg
.kokoro/release/docs.sh
.kokoro/release/publish.cfg
.kokoro/samples-test.sh
.kokoro/system-test.sh
.kokoro/test.bat
.kokoro/test.sh
.kokoro/trampoline.sh
.nycrc
.prettierignore
.prettierrc
CODE_OF_CONDUCT.md
CONTRIBUTING.md
LICENSE
README.md
codecov.yaml
renovate.json
samples/README.md
synthtool > Wrote metadata to synth.metadata.

```
</details>

samples/README.md

@@ -23,6 +23,7 @@
   * [Keyfile](#keyfile)
   * [Oauth2-code Verifier](#oauth2-code-verifier)
   * [Oauth2](#oauth2)
+  * [Verify Id Token-iap](#verify-id-token-iap)
   * [Verify Id Token](#verify-id-token)
 
 ## Before you begin
@@ -231,6 +232,23 @@ __Usage:__
 
 
 
+### Verify Id Token-iap
+
+View the [source code](https://github.com/googleapis/google-auth-library-nodejs/blob/master/samples/verifyIdToken-iap.js).
+
+[![Open in Cloud Shell][shell_img]](https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/googleapis/google-auth-library-nodejs&page=editor&open_in_editor=samples/verifyIdToken-iap.js,samples/README.md)
+
+__Usage:__
+
+
+`node samples/verifyIdToken-iap.js`
+
+
+-----
+
+
+
+
 ### Verify Id Token
 
 View the [source code](https://github.com/googleapis/google-auth-library-nodejs/blob/master/samples/verifyIdToken.js).