rephrase description of adding GUI to USER instruction

Daniel Hauenstein · Daniel Hauenstein · commit a726bf5ab85d · 2016-11-15T13:18:47.000+01:00
Avoiding passive voice in description of adding a GUI to the USER
instruction
diff --git a/engine/reference/builder.md b/engine/reference/builder.md
@@ -1204,8 +1204,8 @@ into the newly created volume.
 
 The `USER` instruction sets the user name or UID to use when running the image
 and for any `RUN`, `CMD` and `ENTRYPOINT` instructions that follow it in the
-`Dockerfile`. Optionaly a group name can be set by adding a `:` and a GID, which will be used as the GID 
-for `RUN`, `CMD` and `ENTRYPOINT` instructions.
+`Dockerfile`. To specify the group ID (GID), add it as a suffix to the UID, 
+separated by a colon. For instance, `MYUSER:MYGROUP`.
 
 ## WORKDIR
 
