README: minor: use smaller logo for fast gh page load
<div align="center">
- <a href="https://iris-go.com"><img src="https://iris-go.com/images/logo.png" alt="iris-go.com logo" height="160"></a>
+ <a href="https://iris-go.com"><img src="https://iris-go.com/images/logo-sm.png" alt="iris-go.com logo" height="160"></a>
# Iris Web Framework <a href="README_ZH.md"><img width="20px" src="https://iris-go.com/images/flag-china.svg?v=10" /></a> <a href="README_GR.md"><img width="20px" src="https://iris-go.com/images/flag-greece.svg?v=10" /></a> <a href="README_ES.md"><img width="20px" src="https://iris-go.com/images/flag-spain.png" /></a> <a href="README_KO.md"><img width="20px" src="https://iris-go.com/images/flag-south-korea.svg" /></a> <a href="README_FA.md"><img width="20px" src="https://iris-go.com/images/flag-iran.svg" /></a>
