Fix typo in README

[dawei-wang]

Fix #131

Proposed changes

Describe the use case and detail of the change. If this PR addresses an issue on GitHub, make sure to include a link to that issue using one of the supported keywords here in this description (not in the title of the PR).

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [x ] I have read the CONTRIBUTING document.
• [ x] I have tested that the NGINX Unprivileged Docker images build correctly on all supported platforms (check out the README for more details).
• [x ] I have deployed the NGINX Unprivileged Docker images on an unprivileged environment and checked that they run correctly.
• [x ] I have updated any relevant documentation (README)