cmd/getgo: display that -i stands for interactive mode
cmd/getgo: display that -i stands for interactive mode

Change-Id: Ia6d8b0e9b2a355d2c4b41634fd51de51187be833
Reviewed-on: https://go-review.googlesource.com/51931
Reviewed-by: Kevin Burke <685caccd7011cfe58b8f9ef79ff9dca388f03c79@inburke.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools