doc/tutorial/kubernetes: support for gnu sed

sed doesn't behave the same on mac os and linux. this change fixes
the behavior by creating a backup file _kube.cue.bak_ for _kube.cue_
and deleting the file after successful completion.

Issue cue-lang#64

Change-Id: I2081f0552f3840cff2b76d690a49bbc54efb5e89
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2661
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>

doc/tutorial/kubernetes/README.md

@@ -359,7 +359,7 @@ EOF
 
 ```
 # set the component label to our new top-level field
-$ sed -i "" 's/component:.*string/component: _component/' kube.cue
+$ sed -i.bak 's/component:.*string/component: _component/' kube.cue && rm kube.cue.bak
 
 # add the new top-level field to our previous template definitions
 $ cat <<EOF >> kube.cue