Update step_four.md

Added a link to the vi text editor website.

Author: guillaumeguerin

engine/getstarted/step_four.md

@@ -46,7 +46,7 @@ commands to run. Your recipe is going to be very short.
         $ ls
         Dockerfile
 
-5. Open the `Dockerfile` in a visual text editor like <a href="https://atom.io/" target="_blank">Atom</a> or <a href="https://www.sublimetext.com/" target="_blank">Sublime</a>, or a text based editor like `vi`, or <a href="https://www.nano-editor.org/" target="_blank">nano</a>.
+5. Open the `Dockerfile` in a visual text editor like <a href="https://atom.io/" target="_blank">Atom</a> or <a href="https://www.sublimetext.com/" target="_blank">Sublime</a>, or a text based editor like <a href="http://ex-vi.sourceforge.net/" target="_blank">vi</a>, or <a href="https://www.nano-editor.org/" target="_blank">nano</a>.
 
 6. Add a line to the file like this: