Update step_four.md (#2629)
Update step_four.md (#2629)

Windows users --like me-- may miss the period, and docker will not find `Dockerfile.txt.`
anweiss/docker.github.io,JimGalasyn/docker.github.io,phiroict/docker,thaJeztah/docker.github.io,anweiss/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,phiroict/docker,docker-zh/docker.github.io,docker-zh/docker.github.io,shin-/docker.github.io,johnstep/docker.github.io,johnstep/docker.github.io,anweiss/docker.github.io,JimGalasyn/docker.github.io,phiroict/docker,docker-zh/docker.github.io,joeuo/docker.github.io,anweiss/docker.github.io,anweiss/docker.github.io,thaJeztah/docker.github.io,docker-zh/docker.github.io,bdwill/docker.github.io,shubheksha/docker.github.io,johnstep/docker.github.io,JimGalasyn/docker.github.io,docker-zh/docker.github.io,shubheksha/docker.github.io,docker/docker.github.io,docker/docker.github.io,shin-/docker.github.io,shin-/docker.github.io,johnstep/docker.github.io,shubheksha/docker.github.io,joeuo/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,shubheksha/docker.github.io,shin-/docker.github.io,joeuo/docker.github.io,shubheksha/docker.github.io,bdwill/docker.github.io,thaJeztah/docker.github.io,bdwill/docker.github.io,JimGalasyn/docker.github.io,thaJeztah/docker.github.io,johnstep/docker.github.io,shin-/docker.github.io,londoncalling/docker.github.io,phiroict/docker,bdwill/docker.github.io,bdwill/docker.github.io,docker/docker.github.io,joeuo/docker.github.io,phiroict/docker,joeuo/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,JimGalasyn/docker.github.io,londoncalling/docker.github.io