Adds example usage in service create command (#62)
Adds example usage in service create command (#62)

* Adds example usage in service create command

 Fixes #61
 Adds two examples to service create command as below

```
Examples:

kn service create mysvc --image dev.local/ns/image:latest

kn service create mysvc --env KEY1=VALUE1 --env KEY2=VALUE2 --image dev.local/ns/image:latest

```

* Adds double spaces in the example text

 Adding double spaces to properly aligns the example
 text while displaying help.

knative/client,knative/client