Update crowdsec.md

[maindust]

Added acceptance of security engine as final step in usage for crowdsec

Please describe the purpose of this Pull Request.

For new Sandbox app/role documentation, please confirm you have completed the following tasks:

• [] App documentation page created
• [] markdownlint reports no errors on edited page(s) - use https://dlaa.me/markdownlint/ if your IDE does not have this built in
• [] Reference added in docs/sandbox/index.md
• [] Reference added in mkdocs.yml nav menu