documentation: replace references to old image names

Created by: das-g

In b0b5ad33 the explicit osmaxx prefix has been dropped. Some parts of the checked-in documentation still mention image names with the prefix, though:

git grep -E 'osmaxx[a-z]+'

docs/project-development-environment.md:docker-compose run osmaxxwebappdev /bin/bash
docs/project-development-environment.md:`docker-compose build --no-cache osmaxxwebappdev`.
docs/project-development-environment.md:docker save osmaxx_osmaxxdatabase > /tmp/osmaxx-database-alpha1.docker-img.tar
docs/project-development-environment.md:docker save osmaxx_osmaxxwebapp > /tmp/osmaxx-webapp-alpha1.docker-img.tar