Typo fix (#217)

Author: RDIL

README.md

@@ -79,7 +79,7 @@ VERBOSE: image name: powershell.local:6.1.0-preview.2-ubuntu-trusty-20180531
 ### Run the Docker image you built
 
 ```sh
-PS /powershell-docker> docker run -it --rm powershell.local:6.1.0-preview.2-ubuntu-14.06 pwsh -c '$psversiontable'
+PS /powershell-docker> docker run -it --rm powershell.local:6.1.0-preview.2-ubuntu-16.04 pwsh -c '$psversiontable'
 
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