fix: mark --display-name required in crtprod.go

[DinoChiesa]

also add two example invocations for the command.

$  ./apigeecli products create  --help       
Create an API product

Usage:
  apigeecli products create [flags]

Examples:
Create an API product, using externally defined operations, with auto approval and access=public:
  apigeecli products create --name "$product_name" \
		--display-name "$product_name" \
		--opgrp "$ops_file" \
		--envs "$env" \
		--approval auto \
		--attrs access=public \
		--token $TOKEN

Create an API product, using externally defined operations, with auto approval and access=public, and a quota of 100/minute:
  apigeecli products create --name "$product_name" \
		--display-name "$product_name" \
		--opgrp "$ops_file" \
		--envs "$env" \
		--approval auto \
		--attrs access=public \
		--quota 100 --interval 1 --unit minute \
		--token $TOKEN


Flags:
...