Fix incorrect path in ENTRYPOINT example

The ENTRYPOINT example uses "/usr/bin/ls" as path, but `ls` is located at `/bin/ls`.

Docker-DCO-1.1-Signed-off-by: Sebastiaan van Stijn <github@gone.nl> (github: thaJeztah)

docs/sources/reference/builder.md

@@ -409,7 +409,7 @@ optional but default, you could use a `CMD` instruction:
 
     FROM ubuntu
     CMD ["-l"]
-    ENTRYPOINT ["/usr/bin/ls"]
+    ENTRYPOINT ["ls"]
 
 > **Note**:
 > It is preferable to use the JSON array format for specifying