Update wp_plugin_small.txt

[securestep9]

Adding to the list the Wordpress plugins with recent Critical CVEs:

• post-smtp (CVE-2023-6875)

Checklist

• I have followed the Contributor Guidelines.
• The code has been thoroughly tested in my local development environment with flake8 and pylint.
• The code is Python 3 compatible.
• The code follows the PEP8 styling guidelines with 4 spaces indentation.
• This Pull Request relates to only one issue or only one feature
• I have referenced the corresponding issue number in my commit message
• I have added the relevant documentation.
• My branch is up-to-date with the Upstream master branch.

Changes proposed in this pull request

Adding to the list the Wordpress plugins with recent Critical CVEs:

• post-smtp (CVE-2023-6875)

Your development environment

• OS: MacOS
• OS Version: 14.1
• Python Version: 3.11