docs: update wait command help message

[sjeandeaux]

Without --health-check or --prometheus, the server http does not start

[jackwotherspoon]

@sjeandeaux Thanks for the PR! 😄

All the docs in our docs/ folder are auto-generated from our help messages.

So instead of updating the docs you will want to update the help message directly...

cloud-sql-proxy/cmd/root.go

Lines 331 to 340 in bfb63f5

	Configuration
	By default, the Proxy will wait up to the maximum time for the startup
	endpoint to respond. The wait command requires that the Proxy be started in
	another process with the HTTP health check enabled. If an alternate health
	check port or address is used, as in:
	./cloud-sql-proxy <INSTANCE_CONNECTION_NAME> \
	--http-address 0.0.0.0 \
	--http-port 9191

[jackwotherspoon]

One small nit, otherwise LGTM 👍