Update image link with explicit format extension

It looks like GitHub Markdown renderer doesn't like image links without
explicit format extensions and <img> inside <a> doesn't link to the <a>
target.

Author: kateinoigakukun

README.md

@@ -24,5 +24,5 @@ If you'd like to track the status of our builds, please refer to our [GitHub Act
 
 
 <a href="https://www.emergetools.com/">
-  <img src="https://github.com/swiftwasm/swift/assets/11702759/6cb83079-f3e0-4749-b40d-a684fae160ad" width="30%">
+  <img src="/assets/sponsors/emergetools.png" width="30%">
 </a>