chore: tweak logo alt in README.md (#7156)

README.md

@@ -1,4 +1,4 @@
-<p align="center"><a href="https://vuejs.org" target="_blank"><img width="100"src="https://vuejs.org/images/logo.png"></a></p>
+<p align="center"><a href="https://vuejs.org" target="_blank"><img width="100" src="https://vuejs.org/images/logo.png" alt="Vue logo"></a></p>
 
 <p align="center">
   <a href="https://circleci.com/gh/vuejs/vue/tree/dev"><img src="https://img.shields.io/circleci/project/vuejs/vue/dev.svg" alt="Build Status"></a>