Fix typo in sidenav for in Cookbook Animation "Animate a widget using a physics simulation“

[WalkQuackBack]

Description of what this PR is changing or adding, and why:

Fixes a missing letter in the sidenav for the text "Animate a widget using a physics simulation".

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[parlough]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 2d1eb34):

https://flutter-docs-prod--pr11234-walkquackback-patch-1-l6t0i5fd.web.app

[parlough]

Nice catch, thanks so much!