[fix] example echo because echo v3

[ttatsato]

Summary

example echo because echo v3.

ref

cannot find package "github.com/labstack/echo/engine/standard" https://github.com/labstack/echo/issues/880

test

test item	test method	test result	date
main.go works fine.	cd examples/chat-echo && go run main.go	🆗	2020-01-11

[olahol]

Fixed now the examples should work with Echo v4.