Typo in path to mastodon logo in README.md (#18632)

Less ass, more assets.

README.md

 <h1><picture>
   <source media="(prefers-color-scheme: dark)" srcset="./lib/assets/wordmark.dark.png?raw=true">
   <source media="(prefers-color-scheme: light)" srcset="./lib/assets/wordmark.light.png?raw=true">
-  <img alt="Mastodon" src="./lib/asssets/wordmark.light.png?raw=true" height="34">
+  <img alt="Mastodon" src="./lib/assets/wordmark.light.png?raw=true" height="34">
 </picture></h1>
 
 [![GitHub release](https://img.shields.io/github/release/mastodon/mastodon.svg)][releases]