Fixed typo in Dockerfile example

Docker-DCO-1.1-Signed-off-by: Rob Cannon <rpcann@gmail.com> (github: robpc)

The 'true' command is located in /bin not /usr/bin in the busybox image.

docs/sources/use/working_with_volumes.rst

@@ -50,7 +50,7 @@ volumes to any container created from that image::
   # RUN-USING:          docker run -name DATA data 
   FROM          busybox
   VOLUME        ["/var/volume1", "/var/volume2"]
-  CMD           ["/usr/bin/true"]
+  CMD           ["/bin/true"]
 
 Creating and mounting a Data Volume Container
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