docs/lando-service.md update suggestions

[tormi]

For clarity, I propose the following updates to docs/lando-service.md:

1. Change FROM devwithlando/php:8.2-fpm to FROM devwithlando/php:8.2-fpm-4 under Using Dockerfiles to make it work.
2. Specify the Dockerfile name in the Dockerfile section as follows:

This Dockerfile is located at /app/php/Dockerfile, which is referenced in the build section above.

[reynoldsalec]

@tormi does the relative path works in the build section?