improve example to set path to local oc binary

The example in the CONTRIBUTING doc adds the locally built
oc binary dir at the end of the PATH.  If the user already has oc
in the path then this command won't override it.  This just
changes the example to add the local build to the beginning of the PATH

Author: pgier

CONTRIBUTING.adoc

@@ -137,7 +137,7 @@ $ hack/env make release
 In order to make use of the binaries from your shell, add the build output
 directory to the `$PATH`:
 ----
-$ export PATH="${PATH}:$( source hack/lib/init.sh; echo "${OS_OUTPUT_BINPATH}/$( os::build::host_platform )/" )"
+$ export PATH="$( source hack/lib/init.sh; echo "${OS_OUTPUT_BINPATH}/$( os::build::host_platform )/" ):${PATH}"
 ----
 
 See more information in https://github.com/openshift/origin/blob/master/HACKING.md#building-on-non-linux-systems[`HACKING.md`]